A-level Physics (7407/7408) 
All older Electronics
Random order collection of every old question I could find	Name:
Class:	

 	
Author:		
Date:		
Time:		
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Q1.A radio telescope suffers interference from local industrial equipment. To reduce the interference it is decided to combine the radio telescope output signal with the signal from a receiver that receives only the interference.
The two signals are shown below.

Before the signals are combined, the interference signals must have the same amplitude. This is achieved by amplifying the radio telescope signal.
(a)     (i)      Calculate the voltage gain needed from this amplifier.
...............................................................................................................
...............................................................................................................
(3)
(ii)     It is decided to use a non-inverting amplifier where the voltage gain can be adjusted from 11 to approximately 100.
Complete the circuit diagram for the non-inverting amplifier and include suitable values for the resistors.

(3)
(b)     The amplified radio telescope signal and the interference signal are added together with a summing amplifier.
(i)      Complete the circuit diagram below for a summing amplifier and include suitable values for the resistors.

(3)
(ii)     Explain how adding the two signals reduces the interference signal in the output.
...............................................................................................................
...............................................................................................................
(2)
(Total 11 marks)
Q2.The diagram shows a logic circuit with three inputs A, B and C.

(a)     Write the Boolean expressions for the signals at the intermediate points D, E, and G in terms of the inputs A, B and C only.
D ....................................................................................................................
E .....................................................................................................................
G ....................................................................................................................
(3)

(b)     Complete the truth table below for the logic signals at the intermediate points D, E and G.
 
 	Inputs	 	Intermediate points	
 	C	B	A	 	D	E	G	
 	0	0	0	 	 	 	 	
 	0	0	1	 	 	 	 	
 	0	1	0	 	 	 	 	
 	0	1	1	 	 	 	 	
 	1	0	0	 	 	 	 	
 	1	0	1	 	 	 	 	
 	1	1	0	 	 	 	 	
 	1	1	1	 	 	 	 	

(5)
(Total 8 marks)
Q3.A student designs a high power lamp system which flashes when the music reaches a certain sound level at a party. The system will automatically switch on when the volume of music received by a microphone, that gives only a low output voltage, exceeds a set level which can be adjusted. 5 Hz pulses generated in the system are then gated through to a driver which controls a lamp.
(a)     Draw a system diagram as a possible plan for this system.

 
 
 (8)
(b)     In which subsystem(s) could
(i)      an op-amp be used?
...............................................................................................................  (2)
(ii)     a potentiometer be used? ............................................................................(1)
(iii)    a MOSFET be used?....................................................................................( 1)
(Total 12 marks)
Q4.Figure 1 shows an inverting op-amp amplifier subsystem.
Figure 1

(a)     (i)      Write in the box the letter that corresponds to the virtual earth point in Figure 1.
 
(1)
(ii)     Explain the meaning of the term virtual earth point.
...............................................................................................................
...............................................................................................................
(2)
(iii)    State the input resistance of this amplifier subsystem.
...............................................................................................................
(1)
(b)     Calculate the value of Rf needed to give the amplifier subsystem a voltage gain of − 47.
........................................................................................................................
........................................................................................................................
(3)
(c)     The amplifier subsystem in part (b) is used to increase the signal voltage from an electric guitar.
The voltage from the guitar to the amplifier input is shown in Figure 2.
Draw onto the lower part of Figure 2 the output signal from the amplifier subsystem.
Figure 2

(4)
(Total 11 marks)
Q5.As part of his project, a student constructs the following logic circuit.

(a)     Write down the Boolean expressions for:
D = ......................................................................................................................
(1)
E = ......................................................................................................................
(2)
(b)     Write down the Boolean expression for Q in terms of D and E.
Q = ......................................................................................................................
(2)
(c)     Complete the truth table below for the logic circuit above.
 
 	A	B	C	D	E	Q	
 	0	0	0	 	 	 	
 	0	0	1	 	 	 	
 	0	1	0	 	 	 	
 	0	1	1	 	 	 	
 	1	0	0	 	 	 	
 	1	0	1	 	 	 	
 	1	1	0	 	 	 	
 	1	1	1	 	 	 	

(4)
(d)     His supervisor suggests that the logic circuit can be simplified. What single logic gate would have the same function as the whole circuit above?
............................................................................................................................
(1)
(Total 10 marks)
Q6.Figure 1 shows a simplified diagram of a road safety system for traffic travelling towards a road tunnel. The tunnel is too narrow for two-way traffic and too low for lorries.
Figure 1

C and L are laser beam sensors placed at different heights on the road just before the tunnel. When a beam is broken, the sensor produces a logic 1.
Cars will break the beam at sensor C only. Lorries will break the beams at both sensor L and sensor C.
M is an electronic message display that tells lorries to take a diversion. The message display lights up when it receives a logic 1.
T is a sensor buried in the road inside the tunnel. It produces a logic 1 when an oncoming car is in the tunnel.
The red stop light R comes on when a lorry is detected or when there is an oncoming car in the tunnel. R will light up when it receives a logic 1.
The green go light G comes on when a car is detected and there are no oncoming cars in the tunnel. G will light up when it receives a logic 1.
(a)     Complete the truth table.   Some of the data has already been entered for you.
 	Input	 	Output	
 	Sensor
T	Sensor
C	Sensor
L	 	Message
display
M	Red
stop light
R	Green
go light
G	
 	0	0	0	 	0	0	1	
 	0	0	1	 	0	0	1	
 	0	1	0	 	 	0	 	
 	0	1	1	 	 	1	 	
 	1	0	0	 	 	1	 	
 	1	0	1	 	0	1	0	
 	1	1	0	 	 	1	 	
 	1	1	1	 	 	1	 	
(4)
(b)     Write the simplest Boolean expression for the red stop light R in terms of T, C and L. 
........................................................................................................................
(2)
(c)     The expression for the green go light G could be written as G =
Draw on Figure 2 the logic diagram for this expression using only NOT, AND and OR gates.
              Figure 2
 
(3)
(Total 9 marks)

Q7.(a)     (i)      An op-amp has two inputs. Label on the diagram the correct name for each input.

(2)
(ii)     Explain how the op-amp works as a comparator, making reference to the two op-amp inputs.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(2)
(b)     The circuit shown below is a simple tester for 9V batteries. When the input voltage, Vin, is below 7 V, all the LEDs should be off. As the input voltage is increased the LEDs come on one at a time until they are all lit.

(i)      Calculate the value of R to make the voltage at point X equal to 7 V.
...............................................................................................................
...............................................................................................................
(2)
(ii)     Calculate the minimum input voltage, Vin, at which all LEDs are lit.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(2)
(c)     The circuit is built using red LEDs. When this circuit was tested with the input voltage below 7 V, all LEDs should have been off but remained on dimly. Explain why replacing the red LEDs with blue LEDs solved this problem.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(Total 10 marks)
Q8.In order to reduce the bandwidth needed for transmission of an audio speech signal, the signal is filtered to remove high frequencies.
(a)     Explain what is meant by the bandwidth of a signal.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(b)     Name the type of filter needed to remove high frequencies.
........................................................................................................................
(1)
(c)     (i)      Draw the circuit diagram of a passive filter to remove high frequencies, using a resistor and a capacitor.
Label the input and the output.

 

 

(2)
(ii)     The resistor in the filter has a value of 1 kΩ.
Calculate the capacitor value required to give a breakpoint frequency of 4.0 kHz.
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
(3)
(d)     The graph shows the response of a different filter to remove high frequencies.

(i)      State how the graph shows that this must be an active filter.
...............................................................................................................
(1)
(ii)     Circle the value closest to the breakpoint frequency of this filter.
 
 	 	30 Hz	100 Hz	200 Hz	1 kHz	

(1)
(iii)    A 2 V, 5 kHz signal is applied to the input of this filter.
Calculate the output signal voltage.
...............................................................................................................
...............................................................................................................
(2)
(Total 12 marks)

Q9.A circuit is required to amplify the output voltage from a microphone by a factor of 100. The input resistance of the amplifier must be 4.7 kΩ to match the internal resistance of the microphone.
(a)     In the space below, complete the circuit diagram of a suitable op-amp amplifier and give suitable resistor values to match the specification. The circuit can be inverting or non-inverting.

(4)
(b)     The op-amp is powered by a ±12 V supply. Assuming an ideal op-amp is used, calculate the maximum amplitude of the input signal before the output becomes saturated.
........................................................................................................................
........................................................................................................................
(2)
(c)     The op-amp has a gain-bandwidth product of 1 MHz. Draw on the graph below how the open loop voltage gain of the op-amp varies with frequency.

(3)
(d)     Calculate the frequency above which the voltage gain of the amplifier is less than 100.
........................................................................................................................
........................................................................................................................
(2)
(Total 11 marks)
Q10.The circuit shown below uses a 2.7 V Zener diode to convert the voltage supply from a USB socket on a PC to power a laser diode module.
 
Specifications for the laser diode module and the USB output are:
Laser diode module current: 88 mA
USB socket supply output: 4.40 V - 5.25 V
(a)     If the voltage from the USB socket is 4.40 V, the current through the Zener diode is 10 mA and the laser diode module is connected, calculate the required value of series resistor, R.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(4)
(b)     State what the Zener diode current will be if the laser diode module is now disconnected from the circuit.
........................................................................................................................
(1)
(c)     Zener diodes are available with ratings of W, W and W. Using a calculation determine which one should be used.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)

(d)     Explain, without using calculations, what will happen in the circuit if the USB socket voltage rises to its maximum of 5.25 V.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)
(Total 11 marks)
Q11.(a)     Complete the circuit diagram below for a difference amplifier with a voltage gain of 100. Label the inputs and output.

(6)
(b)     The difference amplifier is used with strain gauge sensors to measure the strain on a metal girder in a bridge.
A strain gauge sensor consists of very fine wires enclosed in a plastic case, as shown below. When the sensor is stretched its resistance increases and when compressed its resistance decreases. The changes in resistance are very small.

Two strain gauge sensors are glued to the girder, one to the top and one to the bottom, as shown below.

When the girder bends, the resistance of sensor 1 decreases and the resistance of sensor 2 increases.
The sensors are connected to the circuit shown below.

(i)      What is the voltage at point A?
...............................................................................................................
...............................................................................................................
(1)
(ii)     If sensor 1 and sensor 2 are both unstrained and each has a resistance of 200 Ω, what will be the difference in voltage between points A and B?
...............................................................................................................
(1)
(c)     The inputs to the difference amplifier in part (a) are connected to points A and B. When the metal girder bends a small amount, the resistance of sensor 1 decreases by 1% and that of sensor 2 increases by 1%.
Estimate, showing your calculation, the output voltage of the difference amplifier under these conditions.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)
(Total 11 marks)

Q12.A student constructs a circuit from the following logic diagram.

(a)     Complete the truth table below for this logic diagram.
 
 	A	B	C	D	Q	
 	0	0	 	 	 	
 	0	1	 	 	 	
 	1	0	 	 	 	
 	1	1	 	 	 	

(3)
(b)     Write down Boolean expressions for the logic signals at C, D and Q in terms of the inputs A and B.
C = .................................................................................................................
(1)
D = .................................................................................................................
(1)
Q = .................................................................................................................
(2)
(c)     What single logic gate could perform the function of the whole circuit above?
........................................................................................................................
(1)
(Total 8 marks)

Q13.The diagram below shows an op-amp used in an amplifier circuit.
 
(a)     Name the type of amplifier circuit shown. .......................................................
(1)
(b)     Calculate the output voltage Vout when the input voltage Vin = 0.50 V.
........................................................................................................................
........................................................................................................................
(2)
(c)     The input is now connected to a sinusoidal source of rms output 2.0 V and frequency 50 Hz.
(i)      Calculate the peak input voltage.
...............................................................................................................
...............................................................................................................
(ii)     On the axes below draw a trace showing two cycles of the input signal and label it A.
On the same axes, draw the two corresponding cycles of the output signal and label it B.
Add suitable scales to the axes.
 
(6)
(Total 9 marks)
Q14.(a)     Name three different types of medium that a modulated carrier signal could travel through.
1 ........................................................................................................................
2 ........................................................................................................................
3 ........................................................................................................................
(3)
(b)     A block diagram of a radio communication system is shown below. The signals between subsystems are shown.
Label the blank boxes.

(5)
(Total 8 marks)
Q15.A student finds that there is too much high frequency noise (hiss) on the audio frequency signal from a radio receiver he has constructed.
(a)     What type of filter is required to pass the wanted signal and reduce the noise?
........................................................................................................................
(1)
(b)     Draw the circuit diagram of a passive filter that would improve the quality of the signal. Label the input and the output.

 

(4)
(c)     The components he chooses have values of 10 kΩ and 10 nF.
Calculate the breakpoint frequency of this filter.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)
(d)     State with a reason whether this would be suitable for a full range audio frequency signal.
........................................................................................................................
........................................................................................................................
(2)
(Total 10 marks)
Q16.The TDA2030 is an audio power IC amplifier which operates from a maximum supply voltage of ±18 V. The pin connections and a simplified application circuit have been found in the data sheet shown below.
 
 	TDA2030 DATA SHEET

 
(a)     State whether the amplifier is connected in inverting or non-inverting configuration.
Explain your answer.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(b)     C1 and R1 form a filter for the input signal.
(i)      State the type of filter formed by C1 and R1.
...............................................................................................................
(1)
(ii)     Calculate the breakpoint frequency of this filter.
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
(3)
(c)     What is the effect of the inclusion of R3 and C3 in this circuit?
........................................................................................................................
(1)
(d)     State which of the components you could alter to change the overall gain of the system.
Explain your answer.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(e)     State two factors which limit the output power that can be obtained from a circuit made using this IC.
........................................................................................................................
........................................................................................................................
(2)
(Total 11 marks)

Q17.A block diagram of a generalised communication system is shown below.
The signals between subsystems have been numbered.
 
(a)     The type of signal at number 1 is an information signal and at number 3 it is a modulated carrier wave.
State the type of signal that could be at
(i)      2 .................................................................................................................
(1)
(ii)     4 .................................................................................................................
(1)
(iii)    5 .................................................................................................................
(1)
(iv)    6 .................................................................................................................
(1)
(b)     Name a subsystem in the diagram above which could contain
(i)      a diode .......................................................................................................
(1)
(ii)     an oscillator ................................................................................................
(1)
(iii)    a loudspeaker ............................................................................................
(1)
(iv)    a tuned circuit ............................................................................................
(1)
(Total 8 marks)

Q18.Part of a data sheet for a PIN photodiode intended for an optical fibre communication system is shown below.
 
 	Reverse leakage dark current	max 5nA	
 	Sensitivity (reverse current caused by light)	0.6 A/W	
 	Capacitance (at zero bias)	10 pF    	
 	Response time (to 95% amplitude)	5 ns    	

(a)     The PIN photodiode is used in the following circuit which converts an optical signal travelling along a fibre into an electrical signal.

(i)      In what bias direction is the photodiode connected?
...............................................................................................................
(1)
(ii)     Calculate the maximum voltage at X that exists when the photodiode is in the dark.
...............................................................................................................
...............................................................................................................
(2)
(iii)    Sensitivity is the reverse current caused by the light power that hits the active area of the device.
Calculate the current through the photodiode and then the voltage at X when light of power 1μW hits the active area of the photodiode.
Current ..................................................................................................
...............................................................................................................
Voltage at X ..........................................................................................
...............................................................................................................
(3)

(iv)    Calculate the time constant of the circuit assuming the photodiode acts as a simple capacitor.
...............................................................................................................
...............................................................................................................
(2)
(v)     Suggest a reason why the response time of the photodiode given in the datasheet is much less than the value you have calculated in part (iv).
...............................................................................................................
...............................................................................................................
(1)
(vi)    The output voltage at X is low even when an optical signal is present. Suggest two ways of increasing the output voltage.
...............................................................................................................
...............................................................................................................
(2)
(b)     The power of the optical signal received at the photodiode, after passing through the optical fibre, is found to be less than the transmitted power.
Describe and explain two possible causes of this.
........................................................................................................................
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(5)
(Total 16 marks)

Q19.A student is designing a simple testing device to check that metal bars used in a factory are cut to a set length. The device uses three sensitive switches, A, B, and C, which give a logic 1 when pressed.
The bar is placed in the device and pressed against switch A.

(a)     The device operates from a 9 V power supply.
(i)      State what is meant by logic 1 in this system.
...............................................................................................................
(1)
(ii)     Complete the circuit diagram below, adding a resistor and switch so that the output gives a logic 1 when the switch is pressed.

(2)
(b)     If the bar is the correct length when placed against switch A, it presses switch B, but not switch C.

There are three outputs:
L is high if the bar is too long
R is high if the bar length is the right length
S is high if the bar length is too short.
If the bar is not pressed against switch A, all outputs are low.
Write Boolean expressions for the outputs, in terms of A, B, and C.
L = .................................................................................................................
(1)
R = .................................................................................................................
(1)
S = .................................................................................................................
(1)
(c)     Draw a logic diagram, using any logic gates, to show how output R can be produced from inputs A, B and C.

(3)
(d)     Convert your logic diagram in part (c) to one that uses only NOR gates, drawing the converted system with the smallest number of gates possible in the space below.

 

 
(3)
(Total 12 marks)
Q20.(a)     A radio transmitter system consists of the four subsystems.
Label the diagram below with the names of each subsystem.

(4)
(b)     (i)      Which one of the subsystems above produces an unmodulated rf signal and may contain a tuned circuit?
...............................................................................................................
(1)
(ii)     The tuned circuit contains a 5 pF capacitor and a 0.1 μH inductor. Calculate the frequency of the signal that the subsystem produces.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(3)
(c)     A DAB transmitter has a frequency of 227.36 MHz. Calculate the length of a half-wave dipole that would be suitable for use as an aerial for this transmitter.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(Total 10 marks)
Q21.Modern UK passports contain a Radio Frequency Identification Device (RFID) chip connected to a coil of wire.

(a)     The RFID chip operates at a frequency of 13.56 MHz. The RFID chip has an effective capacitance of 20 pF in parallel with the coil.
Calculate the required inductance of the coil.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(4)
(b)     Calculate the length of a half wave dipole aerial for this frequency. Explain why a coil of wire is used at the immigration control desk for reading the data on the RFID instead.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)
(c)     The quality factor, Q, of a tuned circuit is .
If the bandwidth, Δf, of the tuned circuit in a passport is 100 kHz, calculate the quality factor of the tuned circuit.
........................................................................................................................
(1)
(d)     Assume the bandwidth given in part (c) represents the highest bit rate that can be used to transfer data from the RFID. Estimate, using a calculation, the length of time it would take to read 1 KB of data.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(Total 10 marks)
Q22.Stereo music recordings are made by having two separate microphones, one to the left and one to the right of the musicians. For these signals to be transmitted by radio they have to be processed so that a listener with a mono radio receives all of the information, while a listener with a stereo receiver can receive both the left and right channel signals separately.
(a)     For the mono radio listener, the left and right signals are added together and transmitted normally.
Draw the circuit diagram for an op-amp circuit that can add together two audio signals.

 

 
(3)
(b)     The magnitude of the voltage gain of the summing circuit is 1. On your diagram for part (a) mark suitable resistor values.
(2)
(c)     So that the two separate channels can be obtained for the stereo listener, the left and right signals are subtracted from each other, and this information is also transmitted but in a way that cannot be heard by the mono listener.
Draw the circuit diagram for an op-amp circuit that can subtract one signal from the other.

 

 
(3)
(d)     The magnitude of the voltage gain of the subtraction circuit is 1. Mark on your diagram in part (c) suitable resistor values.
(2)
(e)     The stereo radio receives two signals, L+R and L– R. Explain how the left and right signals can be extracted from these combined signals.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(Total 12 marks)
Q23.A student designs an electronic system to control a ventilation fan for a greenhouse. The fan should be switched on only when both the temperature and humidity exceed certain levels that can each be set independently.
(a)     Choosing appropriate input, process and output subsystems from the list below, draw a labelled block diagram to show a possible design for the system.
Choose from:
 
 	AND gate	comparator	driver	humidity sensor	
 	fan motor	temperature sensor	voltage divider	 	

 

 
 
(7)
(b)     In which subsystem would:
(i)      a MOSFET be used .................................................................................................
(1)
(ii)     an op-amp be used ..................................................................................................
(1)
(iii)    a thermistor be used? ..............................................................................................
(1)
(c)     The controller circuit operates from a 12 V power supply and draws a current of 25 mA under all conditions.
The fan motor requires a current of 450 mA when switched on and operates from the same 12 V power supply.
Calculate:
(i)      the total current drawn by the whole system when the fan motor is switched on
...............................................................................................................
(1)
(ii)     the input power to the whole system when the fan motor is switched on.
...............................................................................................................
(2)
(Total 13 marks)
Q24.A system to detect when the temperature in a room exceeds 20°C is needed as part of an energy saving scheme. Part of the system is shown below.

On the diagram above name and label the device used as a
(a)     (i)      temperature sensor
(1)
(ii)     comparator.
The comparator input Y requires a reference voltage of 4 V.
(1)
(b)     (i)      Draw two components and their connections to the circuit to show how this is achieved.
(2)
(ii)     Select suitable values for these components and mark these on the circuit diagram next to each component.
(1)
(c)     The temperature sensor data sheet gives the following graph.

Calculate the value of R that would make the comparator switch at 20°C.
........................................................................................................................
........................................................................................................................
(3)
(d)     Give the output voltage from this circuit when the voltage at X is greater than the voltage at Y
(i)      for an ideal comparator ........................................................................
(1)
(ii)     for a real comparator ............................................................................
(1)
(Total 10 marks)
Q25.A stable power supply voltage of 4.7 V at a maximum current of 100 mA is needed to power an MP3 player in a car.
The problem is that the car battery voltage can vary from as low as 10 V when the car is started on a cold day, up to 14.4 V when it is charging as the engine is running. The circuit below is designed to produce a constant 4.7 V output.

(a)     (i)      What is component X?
...............................................................................................................
(1)
(ii)     What voltage rating should be chosen for X?
...............................................................................................................
(1)
(iii)    In which bias direction is component X placed?
...............................................................................................................
(1)
(b)     The minimum current through component X is 5 mA. The maximum output current from this circuit is 100 mA.
R must be calculated when the value of the input voltage is at its lowest.
Calculate
(i)      the total current flow through R under these conditions
...............................................................................................................
(1)
(ii)     the voltage across R when the input voltage is at its lowest
...............................................................................................................
(1)
(iii)    the required value of R.
...............................................................................................................
(2)
(iv)    Which preferred value should be chosen for R if the minimum current through X is not to fall below 5 mA?
...............................................................................................................
(1)
(c)     The preferred value of resistor determined in part (b)(iv) is not available, but a 33 Ω resistor is used. With the circuit powered up, and the engine running, the MP3 player is disconnected. Component X is then found to be hot.
(i)      Calculate the current through component X.
...............................................................................................................
...............................................................................................................
(2)
(ii)     Calculate the power dissipated by component X.
...............................................................................................................
...............................................................................................................
(1)
(Total 11 marks)
Q26.(a)     A simple radio receiver system consists of the following subsystems.
 
 	af amplifier	aerial	detector	loudspeaker	tuned circuit	

Label the diagram below with the subsystems in the correct order.

(5)
(b)     Which subsystem has an input that is
(i)      a narrow range of modulated radio frequency signals ..........................
(1)
(ii)     a wide range of modulated radio frequency signals ..............................
(1)
(iii)    a large amplitude audio frequency signal ..............................................
(1)
(iv)    a small amplitude audio frequency signal? ............................................
(1)
(c)     Describe the function of the detector.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(Total 11 marks)
Q27.The Boolean equation for a logic circuit with inputs A and B and output Q is:
Q = (A . B) + ( . )
(a)     Complete the truth table to show the logic values of the terms below for all the combinations of the inputs A and B.
 
 	A	B			A.B	 . 	Q	
 	0	0	 	 	 	 	 	
 	0	1	 	 	 	 	 	
 	1	0	 	 	 	 	 	
 	1	1	 	 	 	 	 	

(5)
(b)     Complete the diagram below to show how a logic circuit can be constructed that has the same function as the Boolean equation above using two AND gates, two NOT gates, and one OR gate.

 

 
 
(5)
(c)     State the logic function of the complete circuit above.
........................................................................................................................
(1)
(Total 11 marks)
Q28.A temperature sensor input subsystem is shown below.

(a)     The thermistor shown above has a resistance of 45 kΩ at 0 °C, 20 kΩ at 25 °C, and 1 kΩ at 100 °C.
Calculate the output voltage at B at a temperature of 25 °C.
........................................................................................................................
........................................................................................................................
(3)
(b)     The temperature sensor input subsystem is connected to the comparator circuit as shown below.

Calculate and choose values of resistors, in the 1 kΩ to 10 kΩ range, for the circuit that will make the comparator switch at 25 °C. Label these on the diagram.
........................................................................................................................
........................................................................................................................
(3)
(c)     What voltage would you expect from the output of this circuit when:
(i)      the temperature is 20 °C .......................................................................
(1)
(ii)     the temperature rises to 30 °C? ............................................................
(1)
(Total 8 marks)
Q29.A butcher wants to fit an alarm to a deep freeze, which will warn him if there is a danger of damage to stock in the freezer.

The freezer has sensors with the following outputs:
T is logic 1 if the temperature is too high to store frozen food; and logic 0 if the temperature is at or below the required temperature
C is logic 1 if the lid is closed and logic 0 if the lid is open.

A student is asked to produce a logic system to give an output A to operate the alarm (the alarm sounds if A is high). He decides that the alarm should sound if:
the lid is closed and the temperature is too high, or
the temperature is low and the lid is left open.
(a)     He designs a system to implement this function. Write a Boolean expression for the output A, in terms of T and C.
........................................................................................................................
(3)
(b)     Draw a logic diagram for the system, using any type of logic gates.
(5)

 

 
(c)     Using NAND gates only, draw a diagram of a logic system which has the same function as a 2-input OR gate.

 

 
(2)
(d)     Draw a logic diagram for the system in part (b), using NAND gates only. Draw a ring round any redundant gates or re-draw the final system.

 

 
(5)
(Total 15 marks)
Q30.(a)     The demodulator stage in a radio receiver has an output voltage of 10 mV but can only deliver a very small current. This stage is then connected to an af amplifier.
(i)      Explain why the af amplifier should have a high input resistance.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(1)
(ii)     What type of op-amp based circuit should be used for the af amplifier?
...............................................................................................................
(1)
(iii)    The voltage gain of the af amplifier is to be 28.
Draw a suitable circuit in the space below.
Choose and calculate suitable values for the resistors.
Label these components with their correct values on your diagram and label the input and output connections to the circuit.

 

 
(5)
(iv)    Calculate, using data given earlier in this question the output signal voltage from this circuit.
...............................................................................................................
(2)
(b)     The op-amp IC used has a gain-bandwidth product of 1 MHz.
Calculate the bandwidth of this af circuit and comment on its suitability for this application.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)
(c)     The amplified audio signal is then fed to a push-pull output stage using two MOSFETs. Draw a suitable circuit in the space below, label the p-channel and n-channel MOSFETs.

 

 

 

(4)
(Total 16 marks)
Q31.          The diagram below shows a series circuit containing resistors and silicon diodes.

(a)     Calculate
(i)      the voltage between A and C,
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(ii)     the voltage between B and C.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(4)
(b)     A student connects diode D2 the opposite way round to that shown in the diagram above.
State, giving reasons,
(i)      the new voltage between A and C,
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(ii)     the new voltage between B and C.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(3)
(Total 7 marks)

Q32.The circuit shown in Figure 1 uses an ideal op-amp as a voltage comparator. It is used in a frost warning device in which the LED lights when the temperature falls to 0°C.
Figure 2 shows the variation of resistance with temperature of the thermistor.

Figure 1

Figure 2
(a)     Calculate the voltage at the non-inverting input of the op-amp.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(1)
(b)     Calculate the resistance of R1 to allow the output to switch at 0°C.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(c)     When the LED is on, the current through it is 20 mA and the p.d. across it is 2.0 V.
Calculate
(i)      the resistance of resistor R2,
...............................................................................................................
...............................................................................................................
(ii)     the power dissipated in this resistor.
...............................................................................................................
...............................................................................................................
(4)
(d)     In practice, what value of resistor from the E24 series should be used for R2?
........................................................................................................................
(1)
(Total 8 marks)
Q33.(a)     An information signal and a carrier wave are shown on the axes below.
Show how these can be combined to form
(i) an AM signal
(ii) an FM signal.

(i)      AM signal
            
(3)
(ii)     FM signal
            
(3)
(b)     The information signal has a maximum frequency of 3 kHz.
(i)      Calculate the bandwidth of the resulting AM signal.
...............................................................................................................
(1)
(ii)     The maximum frequency deviation of the FM carrier is ± 5 kHz. Calculate the practical bandwidth of the resulting FM signal.
...............................................................................................................
(2)
(Total 9 marks)
Q34.In the circuit shown, an ideal operational amplifier is used as a voltage comparator.
 
(a)     The voltage Vin is steadily increased from 0 V. Calculate Vin when the sign of Vout changes from negative to positive.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(b)     (i)      The op-amp is required to operate an LED. Add to the circuit an LED and its limiting resistor so that the LED lights when Vin is less than the value calculated in part (a).
(ii)     Explain why the LED functions in the position you have drawn it.
...............................................................................................................
...............................................................................................................
(iii)    Calculate the minimum value for the limiting resistor with the LED. Assume that the LED has a voltage drop of 2.0 V across it when emitting and a maximum current of 25 mA through it.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(5)
(c)     An LDR is now connected between the points A and B in the circuit. The characteristic of the LDR is shown below.
 
Determine the light intensity at which the LED switches.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)
(Total 10 marks)
Q35.An electronic dice is to be constructed so that when a push switch is operated, the dice counts very quickly and continuously from 1 to 6. When the push switch is released, the number the dice has counted to, is displayed. The dice will consist of the following subsystems.

The binary counter consists of three rising edge triggered D-type flip-flops. The counter outputs are X, Y and Z. The most significant bit is Z.

(a)     (i)      On the diagram of the binary counter above, show how these flip-flops must be connected to form a 3-bit binary up-counter. Label the input from the astable and the three outputs X, Y and Z.
(5)
(ii)     Only six possible output states are required from the binary counter for the operation of the dice. Add to the diagram of the binary counter the additional connections and components needed to make the counter count from 0 to 5 and then reset on the sixth input pulse.
(3)
(b)     The outputs from the binary counter are to be decoded to operate the display to produce the dice numbers. Complete the table below to show how the dice output is related to the binary counter output.
 
 	Binary counter output	Dice output	
 	Denary	Binary
Z Y X	Dice number	LED on	
 	0	0 0 0	1	D	
 	1	0 0 1	2	A	
 	2	0 1 0	3	D, A	
 	3	 	4	 	
 	4	 	5	A, C,D	
 	5	 	6	 	

(5)
(Total 13 marks)
Q36.(a)     State two characteristics of an operational amplifier.
........................................................................................................................
........................................................................................................................
(2)
(b)     (i)      Draw a circuit diagram showing an operational amplifier used as an inverting voltage amplifier.
(ii)     Give suitable values for the components you have used in the circuit for a voltage amplification of magnitude 150.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(4)
(c)     When negative feedback is used with an amplifier the bandwidth increases.
(i)      Explain what is meant by negative feedback as applied to the circuit drawn in part (b).
...............................................................................................................
...............................................................................................................
...............................................................................................................
(ii)     Give one other advantage of using negative feedback in this application.
...............................................................................................................
(iii)    State what is meant by the bandwidth of an amplifier.
...............................................................................................................
...............................................................................................................
(iv)    Indicate on the graph below, by means of a horizontal line, the bandwidth of the amplifier whose characteristic is shown.
...............................................................................................................
...............................................................................................................
 
(5)
(Total 11 marks)
Q37.The figure shows the circuit of a high-pass filter. The ac source has a variable frequency.
 
(a)     (i)      Calculate the frequency at which the reactance of the capacitor is 1.0 × 103 Ω.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(ii)     Explain why  will have a low value at low frequencies.
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
(4)
(b)     The variation of   with frequency for the high-pass filter is shown below.
 
Explain, without further calculation, the form of the characteristic.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(Total 6 marks)
Q38.Radio frequency identification (RFID) is a system now widely used as an alternative to barcodes. A typical application is in the anti-theft tags used on shop goods.

The aerial in the transceiver transmits a signal which activates the tag. The tag then transmits data to the transceiver. The frequency of the signals is typically 13.56 MHz. The transceiver's aerial is made of a coil of wire, which also acts as the inductor for the tuned circuit.

(a)     Show that a suitable value for the inductor, L, is approximately 920 nH.
........................................................................................................................
........................................................................................................................
(2)
(b)     On the graph below sketch what you would expect the frequency response curve for the tuned circuit to look like.

(2)
(c)     It is important for reliable operation that the RFID system has high selectivity. This can be achieved if the tuned circuit has a high quality factor (Q). With reference to your graph, explain what these terms mean.
selectivity .......................................................................................................
........................................................................................................................
quality factor....................................................................................................
........................................................................................................................
(2)
(Total 6 marks)
Q39.An LDR is being used as a light sensor in a system that will switch on a porch light when it gets dark.
The characteristic for the LDR is shown in Figure 1.
Figure 1

(a)     (i)      Explain how the use of the logarithmic scale in Figure 1 is helpful when displaying this characteristic.
...............................................................................................................
...............................................................................................................
(1)
(ii)     The LDR has a resistance of 60 kΩ when the light level causes the system to switch on the porch light.
State the value of this light level by reading from the graph in Figure 1.
light level.............................. lux
(1)
(b)     Figure 2 shows the circuit for detecting the light level.
The design makes use of an op-amp acting as a comparator.
A red LED acts as an output indicator to aid testing of the detector circuit.
Figure 2

Draw on Figure 2 the connections from points X and Y to the op-amp inputs so that the red LED switches on when the light level falls below the required value.
(1)
(c)     (i)      Calculate the voltage at point X when the red LED switches on.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(2)
(ii)     The reference voltage at Y is produced by two fixed-value resistors.
Calculate the value for resistor R1 in order to achieve the required circuit operation.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(2)
(d)     The red LED was found to stay on dimly even when the light level was well above the value expected to switch it off.
Explain why this might happen and how the problem could be solved.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)
(Total 10 marks)
Q40.A Zener diode is used to produce a stabilized 5.1 V from an unregulated 12 V supply to power a project that requires 80 mA.
Part of the circuit is shown in the diagram.

(a)     Draw on the diagram the Zener diode connected correctly between points X and Y.
(2)
(b)     The Zener diode requires at least 5 mA to maintain its Zener voltage of 5.1 V.
(i)      Calculate the minimum current flowing through R when switch S is closed.
...............................................................................................................
(1)
(ii)     Calculate the voltage across resistor R under these conditions.
...............................................................................................................
...............................................................................................................
(1)
(iii)    Calculate the value of resistor R.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(2)
(c)     The circuit in the diagram above is now constructed using a value of 75 Ω for resistor R.
(i)      Show that the power dissipated in the resistor is approximately 0.6 W.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(2)
(ii)     The project is disconnected by turning switch S off, but the 12 V supply remains connected.
Calculate the current that now flows through the Zener diode.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(2)
(Total 10 marks)
Q41.A fridge is fitted with a temperature-sensing unit to indicate whether the temperature inside the fridge is too high, too low, or at a safe temperature.
The system consists of a temperature sensor that produces a 2−bit binary output, a logic circuit and a low current, common cathode 7−segment display.
Figure 1 shows a block diagram of the system.
Figure 1

Table 1 shows the operation of the system.
Table 1

(a)     Complete Table 2 to show the logic signals required on lines a to g to display the specified characters.
Table 2

(3)
(b)     Circle the single logic gate which would generate the required signal for segment a.
 
 	AND	EXOR	OR	NAND	NOR	NOT	

(1)
(c)     The LEDs in the 7−segment display must be protected by current limiting resistors.
Figure 2 shows two methods, A and B, of connecting current limiting resistors.
Figure 2

(i)      State one disadvantage of method A.
...............................................................................................................
...............................................................................................................
(1)
(ii)     Calculate the value of the current limiting resistors required in method B to limit the current in each segment to 20 mA.
Assume the voltage from the logic circuit is 5 V and the forward voltage drop across each LED in the 7−segment display is 2.2 V.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(2)
(iii)    Circle the appropriate value for these resistors from the following list of E24 resistors.
 
 	 	110 Ω	150 Ω	270 Ω	1.1 kΩ	1.5 kΩ	

(1)
(Total 8 marks)
Q42.          (a)     State the three factors upon which the capacitance of a parallel plate capacitor depends.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(b)     The figure below shows a circuit for measuring the capacitance of a capacitor.

The switch is driven by a signal generator and oscillates between S1 and S2 with frequency f.
When the switch is in position S1 the capacitor charges until the potential difference across it is equal to the supply emf. When the switch moves to position S2 the capacitor discharges through the microammeter which has a resistance of 1000 Ω.
In one experiment a 0.047 µF capacitor is used with a 12 V supply.
(i)      Calculate the charge stored by the capacitor when the switch is in position S1.
(ii)     Calculate the time for which the switch must remain in contact with S2 in order for the charge on the capacitor to fall to 1% of its initial charge.

(iii)     Assuming that the capacitor discharges all the stored charge through the microammeter, calculate the reading on the meter when the switch oscillates at 400 Hz.

 
(6)
(Total 8 marks)

Q43.(a)     (i)      Without drawing circuit diagrams, explain the difference between positive feedback and negative feedback, in the case of an operational amplifier.
For each feedback, state how the gain of the amplifier is affected when compared with the open loop gain.
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
(ii)     Give two advantages of using negative feedback in the case of an audio amplifier.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(5)
(b)     (i)      Draw a circuit diagram of a non-inverting operational amplifier in which negative feedback is used. Label the input voltage and the output voltage. Do not show power supplies.

 
 
(ii)     Suggest suitable values for the components used in the circuit so that a voltage gain of 12 is obtained.
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
(4)
(Total 9 marks)
Q44.In this voltage comparator circuit the LED lights up only when the illumination falling on the LDR in the resistor network lies between two particular values.

The LDR has the characteristic shown below.

(a)     Give the voltage output values of the operational amplifiers which make a current flow through the LED so that it lights up.
........................................................................................................................
(1)
(b)     Calculate the voltages at points X and Y in the resistor chain.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(c)     Use the answers to part (b) to determine the two values of the illumination between which the LED will be lit.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(4)
(d)     Explain how the circuit works, making appropriate reference to your calculations in parts (b) and (c).
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(4)
(Total 11 marks)
Q45. 

In the circuit shown, an input of +1.2 V is applied simultaneously to each of the inputs A, B and C.
(a)     Determine the current flowing through each of the input resistors and mark on the diagram the direction of each current.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(b)     Determine the value of the output voltage, Vout, if Rf = 10 kΩ.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(c)     If Rf is changed to a resistor of 20 kΩ state, with a reason, the value of Vout.
........................................................................................................................
........................................................................................................................
(Total 6 marks)
Q46.The circuit in Figure 1 is that of an operational amplifier being used as an inverting amplifier.
The power supply to the amplifier is ±15 V.
 
Figure 1
(a)     (i)      Calculate the voltage gain for the amplifier circuit.
...............................................................................................................
...............................................................................................................
(ii)     The input signal is sinusoidal of peak voltage 1.0 V. The first graph in Figure 2 shows the variation of the input signal. Sketch on the second set of axes the corresponding form of the output signal. Indicate values on the voltage axis.
 
Figure 2
(5)
(b)     The feedback resistor is now changed for a resistor of resistance 600 kΩ. Sketch a graph on the axes in Figure 3 showing the new output signal, for the same input waveform.
Indicate values on the voltage axis.
 
Figure 3
........................................................................................................................
........................................................................................................................
(2)
(Total 7 marks)
Q47. 

(a)     For the circuit shown calculate the voltage
(i)      at the point A,
...............................................................................................................
...............................................................................................................
(ii)     at the point B.
...............................................................................................................
...............................................................................................................
(3)
(b)     Describe, with a reason in each case, the state of each LED when Vin = 6.5 V.
(i)      LED P ...................................................................................................
...............................................................................................................
...............................................................................................................
(ii)     LED Q ...................................................................................................
...............................................................................................................
...............................................................................................................
(4)
(c)     Determine the resistance of R needed to limit the current through either LED to 20 mA. Assume that the forward voltage drop across either LED is 16.V
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(Total 9 marks)
Q48.Figure 1 shows the circuit of a summing amplifier which uses an operational amplifier with negative feedback. The power supply to the operational amplifier is ±12 V.
 
Figure 1
(a)     (i)      State what is meant by negative feedback. Explain how this is achieved in the above circuit.
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
(ii)      Give two reasons for using negative feedback in an amplifier.
...............................................................................................................
...............................................................................................................
...............................................................................................................
(4)
(b)     The input voltages to the amplifier in part (a), V1 and V2, vary with time according to the graphs shown in Figure 2. Given that R1 = 40 kΩ, R2 = 20 kΩ and Rf = 40 kΩ show on the third set of axes the variation of Vout with time. Indicate values of Vout on the axis.
 
Figure 2
........................................................................................................................
........................................................................................................................
........................................................................................................................
(4)
(Total 8 marks)

 
M1.(a)     (i)      Correct formula ✓,
substitution ✓,
calculation, 50 ✓,
3
(ii)     Correct circuit (non-inverting) ✓,
Variable element in a correct place ✓,
Appropriate values (1kΩ - 10MΩ) (gain of 10 to ≈100) (must work as an amplifier) ✓
3
(b)     (i)      Correct circuit (summing amp) ✓,
Appropriate resistor values (1kΩ - 10MΩ) ✓,
Appropriate gain (0.1 - 3) ✓
3
(ii)     Signals out of phase, inverted ✓,
so when added they cancel ✓
2
[11]
M2.(a)     D = C + B
1
E = 
1
G = 
1
3
(b)     
 	INPUTS	INTERMEDIATE OUTPUTS	
 	C	B	A	D	E	G	
 	0	0	0	0	1	1	
 	0	0	1	0	0	0	
 	0	1	0	1	1	0	
 	0	1	1	1	0	0	
 	1	0	0	1	1	1	
 	1	0	1	1	0	0	
 	1	1	0	1	1	0	
 	1	1	1	1	0	0	

2 marks for each of correct columns D & G
1 mark for column E
5
(Total 8 marks)
M3.(a)     
 
8
(b)     (i)      amplifier ✓        comparator ✓
2
(ii)     set level✓
1
(iii)    driver✓
1
[12]
M4.(a)      (i)      A
(at inverting input)
1
(ii)     a point on the circuit where the voltage is 0v / ground
but not connected to 0v / is almost 0v / simulates 0v
assuming that the op-amp has not saturated
2 max
(iii)    10kΩ (must have units unless 10,000 which assumes standard)
oe 10,000Ω / 10K etc
1
(b)     correct formula rearranged
calculation / substitution
470kΩ
3 for just correct answer with units
3
(c)     inverted,
same frequency,
shape shows evidence of correct gain
maximum amplitude 3v to 5v
4
[11]
M5.(a)     
 
3
(b)     
2
(c)     
 
 	A	B	C	D	E	Q	
 	0	0	0	1	1	0	
 	0	0	1	1	1	0	
 	0	1	0	1	1	0	
 	0	1	1	1	0	0	
 	1	0	0	0	1	0	
 	1	0	1	0	1	0	
 	1	1	0	0	1	0	
 	1	1	1	0	0	1	
 	 	 	 	✓	✓	✓ ✓	

4
(d)     (3-input) AND gate✓
1
[10]
M6.(a)     
 	INPUTS	OUTPUTS	
 	Tunnel Sensor
T	Car Sensor
C	Lorry Sensor
L	Message Display
M	Red Stop light
R	Green
Go light
G	
 	0	0	0	0	0	1	
 	0	0	1	0	0	1	
 	0	1	0	0	0	1	
 	0	1	1	1	1	0	
 	1	0	0	0	1	0	
 	1	0	1	0	1	0	
 	1	1	0	0	1	0	
 	1	1	1	1	1	0	

Column M − 2 marks
Column G − 2 marks
4
(b)     R = T + (C.L)
1 mark for terms
1 mark for OR
Or for full expression S = T +(.C.L)
Max 1 mark
2
(c)     
 
3
(Total 9 marks)
M7.(a)     (i)      Top: non-inverting input✓
Bottom: inverting input✓
2
(ii)     when V+ > V–, output goes high ✓
when V+< V–, output goes low ✓
2
(b)     (i)      5V across 30k + 10k + 10k = 50k (or similar calc) ✓
70k needed✓
2
(ii)     calc leading to✓≥ 9V✓
2
(c)     V-sat output from op-amp is higher✓ than Vf for red LEDs✓
Vf for blue LEDs higher ✓    (Max 2)
2
[10]
M8.(a)     range of frequencies in signal ✓
reference to frequency at which signal drops by
e.g. power 3dB (50%) / voltage 6dB (71%) ✓
2
(b)     low pass / treble cut ✓
1
(c)     (i)      RC filter circuit, with input & output labelled ✓
correct R & C positions ✓
 
2
(ii)     substitute values into f = 1 / 2πRC ✓
rearrange for C ✓
40 (39.7) nF ✓
3
(d)     (i)      gain > 1 ✓
1
(ii)     100Hz ✓
1
(iii)    gain = 0.2 ✓ output = 0.4V ✓
2
[12]
M9.(a)     Feedback resistor from output to –ve input
Correct inverting or non-inverting amplifier circuit
For inverting, 4.7kΩ and 470kΩ in the correct place
For non-inverting, 4.7Ω input resistor AND two resistors in ratio of 99 / 100
4
(b)     Formula
120mV
0.12V
(No unit error if obvious)
2
(c)     

Positive gradient = no marks
Correct intercept on vertical axis
Correct intercept on horizontal axis
Straight line joining
3
(d)     (No possible ecf from (c))
Calculation
10kHz
2
[11]
M10.(a)     10mA + 88mA = 98mA✓; V across resistor = 4.4-2.7 = 1.7V✓;    R=1.7 / 0.098 ✓= 17.3Ω✓
4
(b)     98mA✓
1
(c)     P = I.V = 0.098 × 2.7✓ = 0.265W✓; use 0.5W✓
3
(d)     Voltage across R will increase✓,
so current through R will increase. ✓
current through zener will increase ✓
3
[11]
M11.(a)     Feedback resistor to inverting input
Series inverting input resistor
Series non-inverting input resistor
Resistor from non- inverting input to 0V – same value as Rf
All resistors in range 1kΩ to 1MΩ
Must have negative feedback then ratio of gain resistors = 100
6
(b)     (i)      3V
1
(ii)     0V
No difference
The same
1
(c)     e.g.
New resistor values of 198Ω and 202Ω (1% change)
 
New output voltage = 0.03 × 100 = 3V
3
[11]
M12.(a)     
 
 	A	B	C	D	Q	
 	0	0	1	0	0	
 	0	1	1	1	1	
 	1	0	1	1	1	
 	1	1	0	1	0	
 	 	 	✓	✓	✓	

3
(b)     C = 
D = A+B✓
Q =  ✓ (A + B) ✓
4
(c)     EXOR✓
1
[8]
M13.(a)     (i)      inverting (amplifier) (1)
1
(b)     use of Vout = (–)× Vin (1)
                 = (–)× 0.5 = –2.0 V (1)
2
(c)     (i)      Vpeak (input) = 2.0 × √2 = 2.8(3) V (1)
(ii)     input trace (A): sinusoidal with T = 20 ms (1) 
                                    and peak = 2.8 V (1)
for output voltage, Vpeak (out) = (–)× 2.8(3)= (±)11.3 (V) (1)
            (allow C.E. for value of Vpeak (input) from (i)
trace B: inversion w.r.t. trace A (1) 
            same period as trace A (1) 
            flat region (saturates) at ± 5 V (1)
max 6
[9]
M14.(a)     (any order)
1        free space✓
2        wires (twisted pair, coaxial etc.) ✓
3        fibre✓
3
(b)     
 
5
[8]
M15.(a)     Low pass filter✓
1
(b)     

4
(c)     use of 1 / 2RC✓ = 1 / 6.28 × 104 × 10-8 ✓= 1.6 kHz✓
3
(d)     not suitable✓    cuts off frequencies from too low a frequency✓
2
[10]
M16.(a)     connected in non-inverting configuration ✓
pin 1 is used as input ✓
2
(b)     (i)      high pass / bass cut✓
1
(ii)     use of f0 = 1 / 2πRC✓
1 / 2×3.14159×1×10-6×22×103✓
7(.25) Hz✓
3
(c)     reduce high frequency response / provide a load at hf / shunt high frequencies to 0V / stability / stop oscillation etc✓
1
(d)     R2✓, R4 (allow C2) in feedback circuit ✓
(or ratio between✓ R2 and R4✓)
2
(e)     Load / loudspeaker impedance (allow resistance) ✓
size of power supply voltage (allow heatsink considerations)✓
2
[11]
M17.(a)     (i)      unmodulated carrier wave / sine wave / blank carrier etc✓
1
(ii)     electromagnetic signal / modulated radio wave / ray in fibre etc✓
1
(iii)    modulated carrier wave✓
1
(iv)    information signal / recovered information signal / baseband signal etc✓
1
(b)     (i)      demodulator (could also be modulator)✓
1
(ii)     carrier generator(may also be demodulator)✓
1
(iii)    output transducer✓
1
(iv)    carrier generator / transmitter / receiver✓
1
[8]
M18.(a)     (i)      reverse✓
1
(ii)     use of V = IR✓
5×10-9 × 106 = 5×10-3V or 5mV✓
2
(iii)    use of current = sens × power✓
6 × 10-7A or 0.6 μA✓
voltage = 6 × 10-7A × 106 = 0.6V✓
3
(iv)    T = RC 106 × 10×10-12 ✓= 10-5s or 10μs✓
2
(v)     capacitance given at zero bias, reverse bias decreases diode capacitance / use of smaller resistance than stated in question ✓
1
(vi)    Increase R ✓
use op-amp✓
2
(b)     Attenuation✓ due to absorption✓ and / or scattering of signal in fibre✓Radiation✓ due to signal loss from tight bends or fibre misalignment ✓
5
[16]
M19.(a)     (i)      9V / >4.5V / high voltage✓
1
(ii)     resistor in series with switch from 0V✓
switch between 9V and output✓
2
(b)     L = A.B.C (or A.C) ✓
R = A.B.
S = A  (or A.) (allow)
3
(c)     
 
3
(d)     
 
3
[12]
M20.(a)     
 
4
(b)     (i)      carrier generator✓
1
(ii)     use of f = 1 ÷ 2π √LC✓
 
225 MHz✓
3
(c)     calc leading to λ = 1.32m✓
1.33 ÷ 2 = 0.66m✓
2
[10]
M21.(a)     use of f = 1 / 2π√LC, change subject to L = 1 / 4π2f2C
substitute values, calculation , leading to 6.9μH ✓✓✓✓
4
(b)     use of λ = c / f, substitute values leading to 22.1m ✓
dipole = 11.05m ✓
too large for desk operation ✓
3
(c)     13.56 / 0.1 = 136✓ (could be rounded down to 135)
1
(d)     1KB = 8192 bits (allow 8000)✓
8192 / 100000 = 0.082s
(or allow values based on 8000, 0.08s) regardless of these variations, time to download centres on 80ms✓
2
[10]
M22.(a)     Two input resistors to the inverting input ✓,
feedback resistor to the inverting input from the output ✓,
non-inverting input to 0V ✓
3
(b)     All resistors the same value ✓,
1kΩ<R<4MΩ ✓
2
(c)     Two input resistors, one to each op-amp input ✓,
feedback resistor to the inverting input from the output ✓,
resistor from non-inverting input to 0V ✓
3
(d)     All resistors the same value ✓,
1kΩ<R<4MΩ ✓
2
(e)     (L + R) + (L - R) = 2L ✓,
(L + R) - (L - R) = 2R ✓
or equivalent by diagram or description
2
[12]
M23.(a)     

7
(b)     (i)      driver✓
1
(ii)     comparator✓
1
(iii)    temperature sensor✓
1
(c)     (i)      25 + 450 = 475mA✓
1
(ii)     12V × 475mA✓
= 5.7W✓
2
[13]
M24.(a)     

5
(b)     

5
(c)     thermistor 20kΩ at 20°C✓
2 : 1 ratio (or calc) ✓
R = 10kΩ✓
3
(d)     (i)      0V✓
1
(ii)     >0 – 3V✓
1
[15]
M25.(a)     (i)      zener diode✓
1
(ii)     4.7V✓
1
(iii)    reverse✓
1
(b)     (i)      5 + 100 = 105mA✓
1
(ii)     10 - 4.7 = 5.3V✓
1
(iii)    5.3 ÷ 0.105✓ = 50.5Ω✓
2
(iv)    47Ω✓
1
(c)     (i)      14.4- 4.7 = 9.7V✓     9.7 ÷ 33 = 294mA✓
2
(ii)     4.7 × 0.294 = 1.38W✓
1
[11]
M26.(a)     

5
(b)     (i)      detector✓
1
(ii)     tuned circuit✓
1
(iii)    loudspeaker✓
1
(iv)    af amplifier✓
1
(c)     obtains af signal from modulated wave OR
rectifies modulated carrier wave
filters out rf signal
passes af signal                                                                     Max 2 ✓✓
2
[11]
M27.(a)     
 
 	A	B			A . B		Q	
 	0	0	1	1	0	1	1	
 	0	1	1	0	0	0	0	
 	1	0	0	1	0	0	0	
 	1	1	0	0	1	0	1	
 	 	 	✓	✓	✓	✓	✓	

5
(b)     
 
5
(c)     EXNOR ✓
1
[11]
M28.(a)     Choice of 20kΩ✓
pot div calculation✓
8V✓
3
(b)     upper half the value of lower✓
in range 1kΩ to 10kΩ✓
preferred values 1kΩ and 2kΩ, or 1.8kΩ and 3.6kΩ etc✓
3
(c)     (i)      +12V or high✓
1
(ii)     0V or low✓
1
[8]
M29.(a)     A = C.T✓ +✓ 
3
(b)     

5
(c)     

2
(d)     

5
[15]
M30.(a)     (i)      so it does not load the demodulator✓
1
(ii)     non-inverting amplifier✓
1
(iii)    
 
5
(iv)    10mV × +28 ✓ = 280mV✓
2
(b)     (1 × 106) ÷ 28 = 35.7kHz✓ ✓
suitable for audio sigs (max 20kHz) ✓
3
(c)     push-pull source follower diagram✓
correct n channel symbol upper✓
correct p channel symbol lower✓
diode / resistor biasing ✓
4
[16]
M31.          (a)     (i)      pd across resistors = 12 – 1.4 = 10.6 (V) (1)
pd across 3 kΩ = 10.6 ×  = 4.0 (V) (1)
pdAC (= 12 – 4) = 8.0 V (1)
(ii)     pdBC = 8.0 – 0.7 = 7.3 V (1)
(allow C.E. for value of pdAC from (i))
4
(b)     (i)      diode D2 in reverse mode,  no current flow (1)
all pd dropped across D2 (1)
pdAC = 12 V (1)
(ii)     pdBC = 12 V (1)
max 3
[7]

M32.(a)      = 8 V (1)
1
(b)     at 0 °C, Rth = 100 (Ω) (1) 
R1 = (2 × Rth) = 200 Ω (1)
2
(c)     (i)      R2 =  (1)
= 500 Ω (1)
(ii)     (use of P = I2 R gives) P = (20 × 10–3)2 × 500 (1) 
                                        = 0.20 W (1)
        (allow C.E. for value of R2 from (i))
4
(d)     510 Ω
(allow C.E. for value of R2 from (c)(i))
1
[8]
M33.(a)     
 
(i)      
(ii)     
6
 
(b)     (i)      2 × 3 kHz = 6kHz✓
1
(ii)     2(3 + 5)✓ = 16kHz✓
2
[9]
M34.(a)     V– = 12 ×  (1)
      = 7.8 V (1)
2
(b)     (i)      between Vout and 0 V (1) 
(or from +12 V to Vout) 
correct direction and resistor (1)
(ii)     (since Vin Vout = – 12 V (12 V across LED) (1) 
(or alternative)
(iii)    voltage across R = (12 - 2) = 10 (V) (1) 
10 = 25 × 10-3 × R gives R = 400 Ω (1) 
(or alternatively 22 = 25 × 10-3 to give R = 880 Ω)
5
(c)     to switch LED voltage at B = 7.8 (V) (1)
RLDR given by 7.8 =  or
                    RLDR = 25.(3) kΩ
light level = 30 lux (1)
max 3
[10]
M35.(a)     (i)      D to inv. output, ✓
CK to inv. output, ✓
Resets together ✓
label astable input ✓
three labelled outputs ✓
5
(ii)     AND gate, ✓
output to Reset, ✓
inputs from Y and Z ✓
3
(b)     
 
 	Binary counter output	Dice output	
 	Denary	Binary
Z Y X	Dice
number	Lamps on	
 	0	0 0 0	1	D	
 	1	0 0 1	2	A	
 	2	0 1 0	3	D, A	
 	3	0 1 1	4	A, C	
 	4	1 0 0	5	A, C, D	
 	5	1 0 1	6	A, B, C	

one per correct answer    ✓  ✓  ✓  ✓  ✓
5
[13]
M36.(a)     high input impedance 
very large voltage gain          any two (1) (1) 
low output impedance
2
(b)     (i)      circuit diagram to show:    correct feedback and output (1) 
                                        correct inputs (1)
(ii)     Ra ≥ 1 kΩ (1) 
gives Rf = 150 kΩ (1)
4
(c)     (i)      fraction of output fed back through Rf (1) 
is 180° out of phase with input (1)
(ii)     increased stability or less distortion or controlled gain (1)
(iii)    range of frequencies within which voltage gain 
                        does not fall by 1 / √2 or power by 1 / 2 (1)
(iv)    bandwidth given by gain of  = 16 (1) (15.6)
horizontal line at gain = 16 and inside curve
max 5
[11]
M37.(a)     (i)      (use of XC =  gives)
 = 16 × 104Hz (1)
(ii)      low f gives high XC (1)
XC >> resistance 1.0 kΩ (1)
Vout (= IR) or  is low (1)
(or correct usage of potentiometer equation)
4
(b)     (shown in (i) that at low f,  is low)
as f increases, XC decreases and Vout (across R) increases (1)
until ≈ 0 V across XC and Vout = Vin (1)
2
[6]
M38.(a)     (i)      use of fo = 1 / 2π√LC, L = 1 / 4π2f2C✓
L = 1 / 4 × 9.87 × 183.87 × 1012 × 150 × 10-12 = 918nH✓
2
(ii)     

2
(iii)    Selectivity, the ability to respond only to a narrow range of frequencies (channel) ✓
Quality factor, sharpness of curve, or calc f / 2∆f✓
2
(b)     Communication on one channel✓
one way communication✓
2
(c)     Transmission is 106 × 1024 bits s-1, 1024 × 10 bits transmitted for 1K data✓
10240 / 106 × 1024 = 94ms✓
2
[10]
M39.(a)     (i)      Log graph enables a wide range of values to be displayed on the same axis.
Allow − (enables values to be displayed as straight line)
1
(ii)     7 lux
1
(b)     
 
1 mark for connections correct way round
1
(c)     (i)       60 kΩ / (60kΩ + 30kΩ)) × 12V = 8V
Working − 1
Answer − 1
2
(ii)     R1 = 11kΩ to give same value at Y as switching voltage at X (2:1 ratio) (No ecf on value)
Reason / calculation − 1
Answer − 1
2
(d)     The op-amp is not ideal and will saturate above 0V
Saturation − 1
Need to drop voltage
Voltage drop − 1
Acceptable method
Method − 1
3
(Total 10 marks)
M40.(a)     
 
1 mark for Zener symbol
1 mark for orientation
2
(b)     (i)      80mA + 5mA = 85mA
Answer − 1
1
(ii)     12V − 5.1 V = 6.9V
Calculation and answer − 1
1
(iii)    R = 6.9V / 85mA = 81Ω
Calculation and answer − 2
2
(c)     (i)       P = V2 / RP = (6.9 × 6.9) / 75P = 0.64W
Hence P is approx. 0.6W
Calculation and answer − 2
2
(ii)     I = V / R           I = 6.9 / 75           I = 92mA
Calculation and answer − 2
2
[10]
M41.(a)     
 	A	B	a	b	c	d	e	f	g	Display	
 	0	0	0	0	0	1	1	1	0	L	
 	0	1	1	0	1	1	0	1	1	S	
 	1	0	1	0	1	1	0	1	1	S	
 	1	1	0	1	1	0	1	1	1	H	

1 mark for row L
1 mark for row S (both)
1 mark for row H
3
(b)     EXOR gate
1
(c)     (i)      Different combinations produce different brightness
1 Disadvantage
1
(ii)     R=V/I; (5V − 2.2V) / 20mA; 2.8V / 20mA = 140Ω
1 mark for 2.8V drop
1 mark for answer
2
(iii)    E24 = 150Ω
1 mark for answer
1
(8)
M42.          (a)     area of overlap of the plates
B1
separation of/distance between the plates
permittivity/dielectric constant of free space/the
material/dielectric between the plates (condone of the gap)
B1
2
B1 for 1 factor clearly stated
B1 for other two clearly stated
(b)     (i)      Q = VC (any form) or 0.047 µF × 12
(ignoring powers of 10)
C1
5.6(4) × 10−7 C (0.56 µC)
A1
(ii)     time constant = 4.7 × 10−5 s or 0.01 = e−t/RC
C1
         0.01 = e–t/(0.000047) or 0.01 = e –t /47 or = 
C1
         2.2 (2.16) × 10−4 s or 0.22 ms
A1
(iii)     their (i) × 400 (230 (226) µA or 2.3 × 10−4 A if correct)
B1
6
[8]

M43.(a)     (i)      positive feedback:
part [or all] of output fed back in phase with input (1)
negative feedback:
part [or all] of output fed back 180° out of phase with input (1)
positive feedback – gain increased*
negative feedback – gain decreased*
* (1)
(ii)     voltage gain more predictable [or constant]*
distortion or noise reduced*
bandwidth increased*
* any two (1) (1)
(5)
(b)     (i) 
 
potential divider between outputs (1)
correct inputs (1)
(ii)     Ra ≥ 1 kΩ (1)
using gain = 1 +  gives Rf = 11 kΩ (1)
[or value consistent with Ra]
(4)
[9]
M44.(a)     output of op-amp A is +12 V and output of op-amp B is 0 V (1)
(1)
(b)     VX = 12 ×= 6 (1)
VY = 12 ×= 4 V (1)
(2)
(c)     P is Rldr (1) 
i.e. illumination > 50 lux (1) 
P is Rldr (1) 
i.e. illumination > 100 1ux (1)
(4)
(d)     when P V– V+ for A (1) 
output + (1) 
for op amp B, V– > V+, output –, i.e. current (1) 
when P becomes (1) 
i.e. current stops (1)
(max 4)
[11]
M45.(a)     Ia =  = 0.2 mA (1)
Ib = 0.3 mA and Ic = 0.6mA (1)
correct direction of current shown (1)
(b)     current through Rf = 1.1(mA) gives Vout = 1.1 × 10–3 × 10 × 103 = 11V (1)
negative value (1)
(c)     Vout (22 V)> supply voltage [or saturated] (1)
Vout = (–)15V (1)
[6]
M46.(a)     (i)      (use of G = –  gives) G = –  (1)
                                                = - 10 (1)
(ii)     waveform to be: 
          sinusoidal with same frequency as input waveform (1) 
          inverted with respect to input waveform (1) 
          peak value of ± ≈ 10 V (1) 
                              (allow C.E. for incorrect value of G from (i))
5
(b)     waveform to be:
                    clipped (1) 
                    at ≈ ±15 V (1)
2
[7]
M47.(a)     voltage at A =  (1) = 5.3 V
voltage at B =  = 9.3 V (1)
(3)
(b)     (i)      for op-amp. 1, V+ > V– (1)
∴output (saturated) at +12 V, ∴ P is lit (1)
(ii)     for op-amp. 2, V– > V+ (1)
∴ output (saturated) at –12 V, ∴Q is unlit (1)
(4)
(c)     for P, V = IR gives 12 – 1.6 = 20 × 10–3  × R (1)
                                         R = 520 Ω (1)
(2)
[9]
M48.(a)     (i)      negative feedback: part or all of the output is fed back to the input 
                                                                                180° out of phase (1)
achieved through Rf (1)
(ii)     greater stability 
less distortion                any two (1) (1) 
increased bandwidth 
gain predictable
4
(b)     

negative values (1) 
correct 9 V and 12 V (1) 
saturation (1) 
repeated (1)
max 4
[8]

 
E1.This question was also intended to provide candidates with an opportunity to show their knowledge of op-amp amplifiers.
(a)     (i)      Many correct responses were seen, though some candidates confused input and output voltages.
(ii)     For basic bookwork, this was poorly answered. Many candidates used the +ve input instead of the -ve input of the op-amp for the feedback loop.
(b)     (i)      It was worrying to see how many candidates could not draw summing amplifiers. Few gained all three marks and often the resistor values was too low or the gain inappropriate.
(ii)     Many creditworthy responses were seen with candidates recognising that the two interference signals were inverted. However, too many just restated the information in the question and so did not gain credit.
E2.(a)      This part was answered well with a high proportion of candidates gaining full marks.
(b)     The truth table proved to be slightly more demanding, but still allowed a large percentage of the candidature access to some marks.
E3.This year the systems question was restored to the second question in the paper. The question itself followed a format that was introduced several years ago, and continued this year by asking for the location of various components within the system.
(a)     The response to drawing the diagram was good with over 75% of candidates getting half marks or more, this is a better response than last year. The main problem again was the same as last year where candidates were not able to give the extra detail in the area of the comparators; at this level they should be familiar with the need for a voltage divider to provide a reference voltage in the form of the “set level” subsystem. Very few picked up the reference to the low level of output from the microphone which then needed an amplifier. Similar problems were noted in locating the logic gate to gate the pulses from the astable. An improvement was noted where more candidates were able to locate a driver before the AWD.
(b)     This section was well answered except for part (i) where only just over 10% of candidates were able to locate two places where an op-amp might be located, this was related to the previous section where the need for a microphone amplifier was not spotted. In part (ii), two-thirds knew where the potentiometer could be used and pleasingly nearly 90% could locate the MOSFET in part (iii).
E4.(a)      (i)      Most candidates selected the correct option.
(ii)     A large number of answers tended to repeat the word ‘earth’ used in the question, scoring only one point.
(iii)    The vast majority gave the correct answer.
(b)     Many gave the correct answer, but workings showing clearly how it was arrived at were less common.
(c)     Many were able to sketch an inverted wave of the same frequency, several gave evidence of using the gain to calculate a theoretical peak voltage, but few noticed the power supply limitation of the circuit.
E5.This year the simple logic question focussed on a three input system, with the same mix of questions which tests candidates’ skill in converting between logic diagrams, Boolean algebra and truth tables.
(a)     Being able to write the Boolean expression for an inverter was known to some 96% of candidates, and 80% were able to do the same for the NAND gate, a pleasing, if unsurprising result, this was after all, the accessible start to this question.
(b)     Reading the question and doing as asked came into prominence again here; only answers in terms of D and E were required. This caused a drop in response to some 65% of candidates gaining both marks here.
(c)     A pleasing 67% of candidates gained full marks in completing the truth table, no mean feat as there were 24 entries of 1 or 0 required to fill in the table.
(d)     Only 60% spotted that the whole system behaved as an AND gate and got the mark. It was not necessary to state a three-input gate as that was given earlier in the question, but many included this information.
E6.(a)     A high percentage of candidates correctly completed the truth table. The loss of just a single mark was mainly due to the green light being left on when a lorry was detected.
(b)     Just over half of the candidates were able to construct the correct Boolean expression for the red light from the given completed column in the truth table.
(c)     Part (c) was done very well, with most candidates opting for the solution given on the mark scheme. However, a few candidates also produced an equally valid design based on an expansion of the given expression for G.
E7.This question focussed on the action of an op-amp as a comparator, a simple window comparator circuit with three comparators and a voltage dividing chain, ending with a challenging question on the limitations of common op-amps and one way of solving the problems generated by using the properties of modern LEDs.
(a)     Some two-thirds of candidates were able to state the correct names for the op-amp inputs. There is confusion between the correct names and the power supply connections it seems. In part (ii), 40% of candidates gained both marks here, a surprising result since the action of a comparator is central to the specification. A notable minority, nearly 5%, avoided this section altogether.
(b)     The calculations in both parts of this section resulted in less than half of all candidates gaining the two marks in each part, the first part proving more successful than the second, at 44% and 38% respectively. It is worthy of note that 17% of candidates offered no answer to part (ii) at all.
(c)     Again a challenging section at the end of the question, even knowing about the limitations of the saturated output of op-amps is rare, and solving it with the higher Vf of blue LEDs even rarer, just 13% obtained both marks, 6% of candidates offered no answer here.
E8.(a)     A precise meaning of bandwidth, referring to power or voltage, should be known.
(b)     Most were able to name the appropriate filter.
(c)     Some attempted active filters, but the question asked for a passive filter. Most were able to rearrange the formula to find breakpoint frequency, with some power of ten errors.
(d)     Interpretation of the graph was disappointing. Few were able to identify a filter with a gain >1 as active. Many thought that the frequency at which the gain first dropped from maximum was the breakpoint. Many could not read the gain at 5kHz.
E9.(a)    Most candidates made an attempt but it was disappointing to see just how many were unable to correctly draw either an inverting or non-inverting amplifier circuit.
(b)     Many disappointing responses were received. Some who attempted the question could not successfully divide 12V by 100 to give 0.12V.
(c)     A significant number of candidates made no attempt at this section. Of those who did make an attempt, their offerings comprised everything from sine waves, bandwidth curves and logic timing diagrams through to the correct response. The gain-bandwidth product concept perhaps needs to be taught more thoroughly.
(d)     A significant number of candidates again made no attempt at this section. Of those that did, about half made creditworthy attempts. However, the use of units for quantities does not seem to occur to many candidates who just gave answers of 10 000 or 104 and then expected the examiners to interpret that they were Hz.
E10.This question focussed on the operation of a Zener diode voltage regulator, this topic has been historically a difficult one. This year was no exception; there was a declining response progressively through the question.
(a)     This was the best answered section here many scoring at least some marks although just 36% obtained all four marks.
(b)     The basic premise of the operation of a Zener diode is not well understood; just 45% were able to write down effectively where the current goes when the output is disconnected.
(c)     Less than a quarter of candidates were able to support the choice of power rating for the Zener diode with a valid calculation, just 22% gaining full marks, 10% made no attempt to answer at all.
(d)     Just over 3% of candidates could trace the changes in dissipation of the components as the input voltage rose, 8% offered no answer.
E11.(a)    Most candidates produced creditworthy responses with a significant number gaining full marks. A common error was to give resistor values below 1kohm, which would cause errors when the op-amp was producing large output voltages.
(b)     (i)      Many correct responses were seen for this question.
(ii)     Many correct responses were seen for this question.
(c)     The responses from those candidates who attempted this section were pleasing, with a significant number gaining full credit. A common error was for candidates to calculate the new resistances of the sensor and then use these values in the difference amplifier formula instead of calculating the new voltage from the sensor potential divider.
E12.This year the format went back to previous practice of placing the simple logic question first. Candidates in the main appear to have been fairly well prepared for this type of question and a high level of success was noted here.
(a)     The truth table entries were correctly completed by over 80% of all candidates. It was necessary to get each column correct in its entirety for each mark so this is quite an achievement.
(b)     There were many good attempts at complex functions here, but some failed and did not even heed the instruction to restrict the answers to input variables only. Two thirds of candidates gained full credit here, which is worthy of note. Candidates should be encouraged to make full use of brackets when compiling a Boolean algebraic expression for the avoidance of doubt if nothing else.
(c)     Over 80% of candidates correctly spotted the function of the whole circuit, even more than those who correctly wrote down the Boolean expressions so there must have been the sort of intuitive analysis going on here that is only common amongst those who are well practised in this topic.
E13.The answers to part (a) were often correct, but a noticeable number of candidates then lost a mark in part (b) by failing to include a negative sign in the final answer.
The initial calculation in part (c) was done well, although some candidates divided instead of multiplying by . Many marks were lost, however, in the drawing of the relevant wave forms and in labelling the axes. Common mistakes included poor drawing to the relevant scale marked on the voltage axis, marking the time scale in seconds when it was shown in milliseconds, and failing to realise that the output voltage could not exceed the supply voltage thereby producing saturation.
E14.This first question on types of communications media and a generalised communications system was intended to be an accessible question to provide all candidates with an opportunity to make a positive start to the paper.
(a)     Three marks were gained here by over half of all candidates. Some lost a mark by giving two types of copper wire or “radio waves” rather than a medium. Either air or free space was allowed for the mark but a few candidates gave air, vacuum and free space. A few did not understand the term “medium”.
(b)     64% of candidates gained all 5 marks. There were quite a few “local oscillators” in place of “carrier generators” and a few mixers rather than modulators. Some candidates played safe and added aerials and r.f. amplifiers to the transmitter and receiver boxes and gained full credit.
E15.This question on low pass filters included synoptic material. It met with a generally good response with the exception of section (b) as described below.
(a)     This section was answered very well with 84% of candidates gaining the mark.
(b)     46% gained full marks here, this is well known. There were a few high pass filters (gaining partial credit). 37% attempted active filters and some circuits were correct, but this misreading of the question was a careless loss of marks.
(c)     The calculation of the breakpoint frequency was well answered by many candidates, 65% gained full marks. Substitution into the correct formula and few problems with exponents was common. Most gave a correct unit. A few did not know where to begin, nearly 7% offering no answer at all here.
(d)     Sensible answers were given here by most, 63% gained both marks. Different values were suggested for a suitable breakpoint and most gained credit within the reasoning. Many quoted the range of human hearing.
E16.This question on the application of an audio amplifier IC included synoptic material as well as providing a vehicle for examining filters. It met with a generally good response with the exception of two sections as described below.
(a)     This section was answered well with over 60% of candidates gaining both marks. Some candidates however wrongly stated that seeing negative feedback determined the application.
(b)     Generally a good response was noted here. A small number of candidates lost marks by answering that the filter was “active” instead of the type of passive filter required. In part (ii), if candidates chose the correct equation they usually went on to obtain the correct answer and coped well with exponents. Some tried to use the reactance equation from the data sheet.
(c)     There were only a small proportion of good, clear answers for this. Some recognised the effect on stability and some that the components would conduct high frequencies to zero volts. Most thought the components formed a filter (all types being suggested). Decoupling and reducing noise were other common answers.
(d)     Some very good answers were noted here. Some candidates who quoted the gain equation made it clear that R4 was equivalent to RF etc. Others left this uncertain. Some even correctly referred to the reactance of C2.
(e)     Finding two valid factors amongst the answers offered here was found quite difficult. Common incorrect answers were gain, input signal and component tolerance.
E17.This first question on a generalised communications system was intended to be an accessible question to provide all candidates with an opportunity to make a positive start to the paper.
(a)     The correct responses to this question varied from almost 60% to nearly 90%. Many concise, correct answers were noted, but for part (i) some unnecessarily complex descriptions of a carrier wave were seen when just a name was sufficient.
Part (ii) was found the most difficult with some candidates naming or describing a medium rather than giving a possible type of signal.
(b)     Many correct answers were seen for all parts (i) to (iv). In part (i) the demodulator was by far the most common incorrect answer. As might be expected part (iii) was found the most straightforward. As correct answers to part (iv) there were almost as many “carrier generators” as “receivers”. Several candidates incorrectly named the modulator or demodulator. A significant number thought a tuned circuit could be found in an output transducer – perhaps chosen because it was the last subsystem given in the diagram.
E18.This novel question went further than had been done before into optical communication, following the recent trend of developments in this area at several levels. There was also another opportunity for a piece of extended writing in this question which proved an arduous task for some, especially as it was on the outer back cover of the paper. From the response to this, it was obvious that the majority of candidates had time to answer the paper, as most made an attempt here.
(a)     In part (i) most candidates gave the correct answer with only a few “negative bias” or “forward” answers given and not awarded credit. In part (ii) nearly all those who started with V = IR obtained the correct answer. Nearly all gave a correct unit. Those who did not succeed were usually attempting an unnecessarily complex process. In part (iii) many correct answers were seen. Not all those who obtained the correct value for current could find the voltage. Some were given the voltage mark even though they had used the wrong value for current. Most gave the correct units.
Some use of T = 1.1 RC in part (iv) were seen, but many correct answers also. A significant number substituted correct values but made calculation errors. A few gave the unit as FΩ but seconds are preferred for time. Part (v) was found quite difficult. The more able realised that bias was the issue. Many brought in 95% amplitude from the data in the question.
(b)     Some good answers were seen here, but less than 20% of candidates were awarded all five marks with a wide spread of marks lower than the maximum. Some candidates confused the terminology. It was widely thought that TIR occurs for rays incident at more than the critical angle. Many candidates described dispersion and received no credit for the points made.
It is not good practice to refer to optical fibres as wires. (Students who have handled fibres are less likely to do so.) A few talked about “ohmic losses” and “resistance” and lost credit.
The fact that most candidates completed the paper and indeed most of the final question however poor the response, indicated again that there was sufficient time to complete this years’ paper in the allotted time.
E19.This final question as has been the custom before, returned to the topic of logic, which is heavily represented in the specification and so this must be reflected in the examination. The pattern is established now with a more accessible question earlier on in the paper, followed by a more challenging question at, or near, the end of the paper. This year’s question went from the meaning of logic levels and how to generate them, to Boolean algebra as preparations for drawing a logic diagram that would perform the required function.
Finally, simplification of the logic circuit was examined. All but the final section met with a response in the 50% to 60% range, the extension work at the very end was meant to be challenging and proved to be so, only just over 20% of candidates were up to this.
(a)     In part (i) only nearly 60% of candidates could adequately state the meaning of logic 1 in a logic system. Again in part (ii), the requirement here was for some very simple circuit diagram drawing, involving a resistor and a switch. This was answered correctly by just over half of all candidates.
(b)     The response here to the requirement to convert a written description of the operation of the system into Boolean Algebraic expressions was a little better at over 60% gaining all three marks.
(c)     A good response was noted here. Over 65% of candidates could draw the logic diagram and obtain full marks.
(d)     Less than one candidate in four gained all three marks here, but this was a demanding question aimed at the most able, and those with the time to complete it in what must have been for them, a busy hour.
Most candidates again this year completed their answers to this paper and indeed most of them attempted final question indicating that there was sufficient time to complete this years’ paper in the allotted hour.
E20.This question included material on a generalised transmitter system going on to tuned circuits and use of the resonance formula, and finishing with a calculation on the length of a half-wave dipole for a given frequency in the DAB band. The response to this question overall was good.
(a)     The diagram was learnt well by many. Several candidates wanted to put in af and rf amplifiers but only a few of them did this without missing out essential subsystems. Nearly half of all candidates scored full marks here.
(b)     In part (i), the location of the tuned circuit was fairly well known. But in part (ii), more completely correct answers were seen often showing minimal working. Many gained the mark for correctly substituting values. Those who thought pF is 10-9 F did not usually complete their calculation. Some ignored the square root sign although they had written it down, but despite this, over half of all candidates gained all three marks.
(c)     This was the most successfully answered section of this question, nearly 60% gaining full marks. Plenty of correct values were seen rounded off to a sensible number of figures and with a unit. Significant number could not rearrange the wave equation and some found an incorrect answer by dividing the frequency by 2 and giving the answer in metres.
E21.This question based on a current application of RFID ICs provided the platform for some basic work on resonant frequency, dipole aerials, Q-factor and data rate proved difficult for many. The only well answered section was (c) where the formula was given in the question. The heavy mathematical content must have proved difficult here.
(a)     There were some completely correct calculations but also many attempts to use XL = 2πfL and XC = 1 / 2πfC. Some candidates made an error by substituting 20 pF as 20×10–9 and several missed out the unit. Some scored full marks but did not show much working so would have gained little if they had made an error using their calculator.
(b)     This was a higher scoring section with over 40% of candidates showing correct calculations and explanations – though some others did not notice the need for an explanation. Most explanations were to do with space considerations but a few other good answers considered range and security.
(c)     This was well answered. A few candidates were confused by Δ (one wrote that it could not be found on his calculator). Some gave units for Q (none needed) and a few others made numerical errors.
(d)     This was found very difficult, only just over 20% of candidates scored both marks here. The more able who converted bytes to bits usually went on to obtain both marks but most did not make this conversion.
E22.This question combined a new area of the specification with an old favourite and should not have caused candidates difficulty. It had been hoped that section (a) would have been straightforward for all candidates – requiring them to draw the circuit diagram of a summing amp. While there were many correct responses, too many candidates:
•        reversed the inputs to the op-amp
•        did not connect the non-inverting input to 0V
•        had three or more input resistors, etc.
There were also those candidates who drew hybrids of the summing-difference amplifier, hoping for the examiners to pick out the correct bits with which to give them credit.
Section (b) gained better responses than section (a), but candidates frequently gave resistor values which were too low, or did not give a gain of 1 and or did not label all of the resistors.
Section (c) required candidates to draw the circuit diagram of a difference amplifier, and candidates gave better answers for this than they did for the difference amplifier. However there were a significant number of ‘hybridߣ circuit diagrams with the summing amplifier as well as candidates making a null response, possibly indicating that some centres are still not teaching the difference amplifier.
Responses for section (d) were, not surprisingly, similar to those for section (b). This section was more demanding but it was encouraging to see that many candidates were able to deduce how to separate the Left and Right signals from the two combined signals and so gained appropriate credit.
E23.This question followed a format that was introduced several years ago. It continued this year with some practical elements of choice to design a system. As was the case on last years paper some simple calculations were required.
(a)     The response to drawing the diagram was good, except for the issue of those candidates who were not able to give the extra detail in the area of the comparators, at this level they should be familiar with the need for a voltage divider to provide a reference voltage. Other candidates thought the sensors connected directly to the logic gate and lost credit, as last year. Most candidates were able to correctly draw the final stages of the system.
(b)     This section was well answered. The majority of candidates are aware of the correct devices to be used in the required subsystems.
(c)     Many correct calculations were given here, although more candidates were able to calculate the current than were able to calculate the power.
E24.This question focussed on the op-amp and its use as a comparator.
(a)     The temperature sensor and the comparator were labelled correctly by around 95% of candidates, however fewer could name the components used as a thermistor and an op-amp.
(b)     About two thirds of all candidates could draw a potential divider, using realistic values at least in the kilohm range to answer this, although fewer got the ratio (and hence the required 4V) correct.
(c)     Interpretation from the graph and a subsequent calculation with no step-by-step structure proved too much of a challenge for nearly 80% of candidates here. Most knew that there was something significant about 20Ω here and gained a mark for it. Very few demonstrated a sensible way of arriving at 10Ω for the resistor value.
(d)     Only around a third of candidates could answer the two parts of this section correctly, firstly, there is the issue of which way the comparator will switch, then the constraints applied to the real comparator rather than in an ideal situation.
E25.This question about the use of a zener diode contained requirements to calculate and select an appropriate component value, and calculate the power dissipation of the diode. Responses to the early parts were generally good but tailed off through the question as it got more demanding, as expected.
(a)     (i)      Almost 90% of candidates were able to state that the component was a “zener”, it would have been nice to see the word “diode” as well
(ii)     Surprisingly, less than half the candidates were able to give 4.7V as the zener voltage.
(iii)    Nearly three quarters of the candidates were able to explain adequately that the zener diode should be connected in “reverse bias”, the most succinct way to describe this.
(b)     The response as a whole to the calculation and choice of series resistor was quite good; the first three parts (i) to (iii) saw responses at around the 60% correct level. The only part that proved difficult with less than 50% correct responses was the final part where a preferred value had to be selected given the circuit conditions. This type of question has featured several times in the past and while it is encouraging to see improved responses to it, it would be better to see a more positive response to some simple numeric work.
(c)     This section was poorly attempted which is a pity as it models the final part of the design process; it is no good being able to only partly specify a component in a given design. The vast majority could not carry out the simple task of recalculating the current under different operating conditions; although a few more were able to go on to calculate the power dissipated. This type of question will reappear on future papers so it is imperative that this part of the specification and the skills involved are given a thorough treatment in centres.
E26.This first question on a simple radio receiver system was intended to be an accessible question to provide all candidates with an opportunity to make a positive start to the paper. This was more successful in section (a) than in sections (b) or (c).
(a)     Many correct answers were seen here, nearly 70% of candidates scored full marks on this, but it was fairly common to see the detector and tuned circuit the wrong way round; nearly a quarter of candidates made this mistake and scored three marks.
(b)     This section was more demanding, many candidates scored three marks, it was common to put aerial instead of tuned circuit. The first two parts at around 60% and 50% who gained the mark respectively, were more demanding than the final two parts, where over 80% could answer correctly.
(c)     This final section was discriminating, but with many good answers. Nearly half of all candidates got at least one mark here. A significant number described the detector as “selecting frequencies”.
E27.This first question on logic continues to be the opening feature of the paper giving it a familiar look to settle candidates. This question was generally well answered by the majority of candidates who in the main appear to have been well prepared for this type of question. Only a few mistakes were made by candidates largely in the final section.
(a)     The truth table was correctly completed by nearly three quarters of all candidates.
(b)     Two thirds of all candidates drew the correct diagram here, using the specified gates in the correct relationship to each other. The other third either could not connect the gates correctly, or used gates not given in the question.
(c)     This was the most difficult section of this question; just over one third of all candidates offered the correct logic function.
E28.This final question returned to the op-amp theme which has not been well answered in recent years. This year there were not good responses to parts of it.
(a)     The response to this accessible section was predictably good, nearly two thirds of all candidates gaining full marks here. It should be emphasised that all the working should be shown in a calculation.
(b)     The most frequently seen answer here gained only a single mark out of the maximum of three marks available. Most candidates need more input on the use of voltage dividers and working within the practical constraints imposed. Some candidates were not fully conversant with the need to keep the current within set bounds, or even the issue of preferred values despite this last part being supported by the data sheet.
(c)     Less than one third of all candidates gained credit on the last two question parts despite a range of answers allowed for credit due to the issue of real versus ideal output voltages possible. This point should be covered both in theory and practice.
Most candidates this year completed their answers to this paper and indeed most of the final question indicating that there was sufficient time to complete this years’ paper in the allotted hour.
E29.This question dealt with further combinational logic, a topic that was brought into this module with the advent of the new specification. This question again dealt with the issue of converting a logic diagram using any gates into one that used a single type of gate. It proved a good source of marks for many candidates.
(a)     More than half of all candidates were able to write the appropriate Boolean expression, credit was not awarded for long strings of unfactorised terms. It is worth noting here that a few spotted a simpler but more advanced solution, they were of course awarded equal credit.
(b)     Nearly half of all candidates drew the required logic diagram faultlessly and so gained full credit. It is worth noting here that those candidates who spotted the simpler but more advanced solution to the previous section, were of course awarded equal credit for the appropriate diagram that went with it.
(c)     This simpler section met with a surprisingly poor response. It was answered less well than the preceding section despite being of a more accessible nature as it referred back to basic theory and was “stand alone”. This section did not need input from the preceding section, maybe this was not spotted by candidates.
(d)     This was the most demanding section of the question and was attempted by virtually all candidates, but met with the lowest response as was to be expected. Less than a quarter of all candidates gained maximum marks here. Since there will always be a more demanding logic question on this paper, candidates should expect and prepare for it.
E30.This synoptic question on the operation of the non-inverting amplifier and a push-pull output stage using MOSFETs met with a poor response overall. Centres are reminded that the A2 specification includes synoptic content and this application is relevant to the af amplifier and output stage of any radio receiver.
(a)     There were few clear explanations of the effect of loading on an output stage in part (i). “So that it will amplify” was more common than the credit worthy “So that it draws little current”. Less than a quarter of the answers were worthy of the mark Part (ii) was better known, just over half the answers were judged to be correct. Part (iii) gave rise to some good neat diagrams by less than a quarter of candidates, but also many inverting amplifiers, comparators and Schmitt triggers were attempted. The best response on this question was seen to part (iv), the calculation. Almost three quarters could answer this correctly.
(b)     Some correct answers for bandwidth were seen but fewer comments worthy of a mark. Many could not connect the result of the calculation to the suitability for this application. Only just over 20% of the answers were given full marks and over a tenth of all candidates did not even attempt this, the most significant omission of an answer to a section of a question on the paper so far.
(c)     This was generally found to be very difficult with very few candidates showing working bias components. A few junction transistors were used and of those who attempted to draw MOSFETs, very many could not draw the correct symbols in the correct location let alone add some bias components. There were many untidy diagrams here and the marks gained on this question were minimal.
E31.          Answers to this question were very disappointing and showed a lack of basic understanding of the concepts involved. In parts (a)(i) and (ii) the majority of candidates applied the potential divider equation directly to the supply voltage, ignoring the potential drop across the two diodes. This obviously gave the wrong answer, but these candidates were given some credit for the method of working and the answer was carried forward to part (ii), so it was possible to gain some marks there.
The majority of candidates failed to gain any marks in part (b). The idea that the whole of the supply potential would be dropped across the reversed diode was beyond the candidates and V was the usual answer for the pd between B and C.

E32.This was the best answered question on the paper and it was not uncommon for candidates to gain full marks. Again, it was carelessness which produced a loss of marks. Part (a) was answered correctly by almost all candidates. The value of pd obtained in part (a) together with the resistance of the thermistor at 0 °C were used correctly by the majority of candidates to obtain the value of R1.
The common mistake in part (c) was to use the value of the given pd (2.0 V) as the pd across the resistor R2 rather than that across the LED, thus giving the wrong value for R2. Allowing the value of R2 to be carried forward enabled most candidates to calculate the power dissipated in this resistor.
Several candidates made careless mistakes in part (d) when deciding on the resistor to be chosen from the E24 series. Several chose a value lower than that calculated in part (c) and others, when their calculated value happened to be included in the E24 series, chose a different value.
E33.This question was about AM and FM signals and their respective bandwidths. It has appeared in this form several times in the recent past. Candidates should have been well prepared for this.
(a)     Some excellent answers were seen here, but under half of all candidates gained full marks. Many lost marks by drawing careless and untidy diagrams.
(b)     Part (i) was well answered; over half could perform the simple calculation. Part (ii) was more discriminating, less than 30% could provide the correct answer, and it was pleasing to see some answers with the working shown.
E34.A large number of candidates calculated the correct switch over voltage in part (a). In part (b) it was pleasing to see that a majority of candidates had drawn the LED between the output and 0 V or between the output and the 12 V supply. In addition, the direction of the diode was usually correct and also the calculation for the value of the series resistor.
Part (c) proved to be more difficult, but many candidates successfully carried out the calculation giving the resistance of the LDR and subsequently read correctly the light intensity from the graph. Other candidates did not know where to start and made a guess at the resistance. If no effort had been made to calculate the resistance, there was no carry forward error for the value of the intensity.
E35.This question should have provided few difficulties to candidates.
Section (a) part (i) required candidates to connect the D-type flip-flops as a 3-bit counter, which most candidates were able to do successfully. A small minority chose to label the output in reverse, with the Z output being from the flip-flop on the left, presumably as a result of getting confused with the binary number outputs. Part (ii) again provided few problems for most candidates, though there were candidates who chose to gate the X and Z counter outputs together rather than the Y and Z outputs. Presumably this was as a result of the counter being required to have 5 as its maximum value.
Section (b) was well answered by the majority of candidates, though a few had some difficulty in converting 3, 4 and 5 into binary numbers while others struggled to visualise the dice output for 4 and 6.
E36.The choice of two characteristics of an op-amp, in part (a), was slightly disappointing with candidates giving irrelevant information concerning op-amps, such as ‘there are two inputs’ or ‘the signal is reversed if fed into the negative terminal’.
The circuit diagram in part (b) was usually drawn correctly although one candidate drew the circuit for the non-inverting amplifier. Values for the input resistor Ra should be ≥ 1 kΩ. This was usually the case. Calculation of the feedback resistor Rf was correct.
Most of the candidates performed well on the subject of negative feedback in part (c) and could also explain what was meant by bandwidth, but attempts at drawing the corresponding level on the graph were disappointing, with either no attempt being made or else being drawn at the incorrect value.
E37.Answers to this question were, in general, very poor. The calculation in part (a) was meant as a lead in to the next part and to get the candidates thinking in terms of reactance. The majority of candidates calculated the correct frequency in part (i), although there were many instances of significant figure errors. However, in part (ii) the same candidates seemed to forget about reactance and attempted an answer in terms of a high pass filter rather than in terms of the reactance at low frequencies and considering the circuit as a form of potential divider.
The answers to part (b) were, if anything, worse than part (a) and the majority of candidates merely described the shape of the curve without any explanation. As an alternative to this form of answer many candidates again discussed high pass filters simply stating that a certain range of frequencies were either blocked or allowed through.
E38.This question based initially on an application of a tuned circuit, going on to data transfer, met with a mixed response.
(a)     Two-thirds of all candidates managed to show they could calculate the required value for the inductor, either by rearranging the resonance equation and solving for L, which gave 918nH, or by working through the equation using the 920nH to give a frequency slightly below the value of 13.56MHz. Credit was not gained when there was no evidence of a calculation.
The sketch of the frequency response curve was very well answered. In the next part the terms selectivity and quality factor were known collectively by just under half of all candidates, with quality factor receiving the poorer responses.
(b)     Very few (just over 10%) candidates could completely explain half-duplex communication. To offer just one way communication was only enough for a single mark, but even this was only given by just over half who attempted it.
(c)     This calculation of the time it would take to read all the data from the tag proved too difficult for the majority of candidates, a mere 10% gained both marks. Several totally unrealistic answers should have been recognised and the calculation re-worked.
E39.(a)      The mark in part (i) was most often lost by the explanation being too general − e.g. ‘it’s better’. However, the reading required from the graph in part (ii) was accurately achieved by most candidates.
(b)     A significant number of candidates were able to connect the inputs to the correct pins of the op-amp to make the circuit function as described.
(c)     The question appeared to expose the lack of understanding of how a transistor-based control circuit can be used to interact with an independent power circuit via an interface (relay).
(d)     This part follows the established question of looking at how saturation affects the output voltage characteristics of real op-amps and how this differs to the characteristic for an ideal one. Candidates are expected to formulate their answer based on the limitations of the op-amp in comparator mode as given in the question.
The latter part of the answer should have focused on how to make the low output voltage from the op-amp smaller than the forward voltage of the LED rather than just reducing the current. Solutions that would have appreciably affected circuit operation when the op-amp gave a high output or potentially cause damage to the LED did not gain credit.
E40.(a)     An easier start than last year as the position for the component was already given. A large number of candidates were successful here although one or two protective resistors crept in and some symbols had the circle with a free-floating centre character which detracted from the quality of the answer.
(b)     This section of the question was straight forward for the majority of candidates.
(c)     The power calculations proved to be more selective in terms of response. Good candidates spotted the need for a V2 / R approach in (i), whilst the easier calculation in (ii) proved to be a more demanding concept as the calculation needed to be centred on the data for the series resistor R.
E41.(a)     A high percentage of candidates correctly completed the truth table.
(b)     Only a few candidates who achieved success in section (a) did not go on to gain the mark here.
(c)     Answers relating to the dimming of segments as more segments were lit were less evident, but the calculation and selection of the E24 resistor proved to be straight forward.
E42.          (a)     Most candidates gave one factor unambiguously, usually the separation of the two plates. The area factor often stated to be the surface area of the plates or the cross-sectional area of the plates. Relatively few stated that it was the permittivity of the medium between the plates that was important.
(b)     (i)      This was generally done well, only less able candidates ignoring the μ or quoting C = QV.
(ii)     There were many correct answers but this part caused more problems for candidates than part (i). The ‘fall to 1%’ was often interpreted as ‘fall by 1%’, and a significant proportion used the charge calculated in (i) to determine the time constant probably confusing the symbol C with the unit C.
(iii)     Many did not appreciate that this was simply the answer to part (i) × 400, and proceeded to perform more elaborate calculations, often involving the exponential function.

E43.There were many loosely worded answers to part (a)(i) which were not given full credit. Very few answers were given in terms of phase. Most answers dealt with connecting the feedback to the inverting or non-inverting input. Even though this does not really explain the difference between the two types of feedback, some credit was given for such answers. No credit was given to answers which stated that the feedback was connected to the positive or negative inputs of the op-amp. The advantages of using negative feedback were well known.
The majority of circuit diagrams presented in part (b) were drawn correctly, although many candidates unfortunately opted for the inverting op-amp circuit. Although the majority of candidates were aware of the simple relation between Ra and Rf in part (b)(ii), marks were lost by choosing an unsuitable value for Ra and it was disappointing to see so many scripts with values as low as I and I0Ω being given to Ra; the smallest value accepted was I kΩ
E45.This question gave the best results in the paper. Parts (a) and (b) were usually correct and the only common error occurred in part (c), where candidates failed to realise that the output would saturate at –15 V. There appear to be on-going problems at this level in that candidates seem to ignore saturation in operational amplifiers and clipped rectified waveforms across zener diodes.
E46.This was a straightforward question and high marks were scored consistently. Part (a) generally realised maximum marks, although there were many candidates who gave a unit of voltage to the voltage gain and were consequently penalised. The sketch graph was usually drawn correctly with the correct peak value.
Part (b) did not fare as well as part (a) since many candidates did not seem to be aware that the output saturated at approximately 15 V and drew the curve with a maximum at 20 V. Other candidates drew the full sinusoidal curve, with a maximum voltage at 15 V, but no clipping was indicated.
E47.Most candidates performed the calculation of the two voltages in part (a)(i) quite well, either by recalling the potentiometer equation or by calculating the current through the resistor chain and hence obtaining the voltages at A and B. The most significant error occurred when the potential difference between A (or B) and the supply voltage (+ 12 V) was calculated and not corrected to the absolute value at A (or B) with respect to zero volts.
Although many candidates gave the correct logical steps in their reasoning in part (b), they failed to deduce correctly whether or not the LEDs were on. What saddened the examiners was the number of candidates who gave the correct input values to both op-amps but failed to give the correct output voltage. Many candidates showed their lack of understanding of op-amps by adding or subtracting the voltage at A (or B) to the Vin voltage and equating this to the output.
Although the voltage drop across the LED was given in part (c), many candidates ignored this and proceeded to calculate the series resistance on the assumption that the voltage drop across the LED was zero. Alternatively, candidates assumed that 1.6 V was the voltage drop across the resistor.
E48.Answers to part (a)(i) were fairly disappointing in that candidates explained negative feedback in terms of the output being fed back into the negative input, without stating that the output voltage was 180° out of phase with the input voltage. The advantages of using negative feedback in an amplifier were well known.
In part (b) most candidates did not notice that there was a simple relationship between R1, R2 and Rf resulting in a simple equation giving Vout in terms of V1 and V2. The method adopted was to calculate the value of Vout each time. When drawing the form of Vout, some candidates did not include the negative factor and gave Vout as being positive. The other error which occurred was that the maximum value of Vout was given as 15 V, not realising that it saturated at 12 V.