AQA A Level Physics Paper 3 Section B: Electronics June 2025 question paper 7408/3BE

Figure 1 shows a system diagram for a timer designed by a student.

The system displays the numbers from 00 to 59 before resetting. The timer is designed to count in time intervals of 1 s.

Part of Figure 1 is enclosed within dashed lines. Figure 2 shows an incomplete circuit diagram for this part of the system. Figure 2 also includes a switch S that is used to manually reset both counters.

Each counter in Figure 2 advances by 1 when CK receives the rising edge of a pulse. When R on a counter receives a logic 1, that counter resets.

Complete the circuit diagram in Figure 2 by adding:

• logic gates to select the reset code for each counter

• a link to show how the second counter is triggered from the first counter

• a resistor with switch S to form the manual reset

• logic gates for each of the counters to provide the reset conditions at R.

Another student designs a logic system to combine judgements from three people. When at least two of the three people input logic 1, the system provides an output of logic 1. Otherwise, the output is logic 0.

The three people are represented by inputs X, Y and Z.

The output Q from this logic system is given by the Boolean expression:

Complete Figure 3 to show the combinational logic diagram for this expression. Use only the logic gates indicated in the expression. Do not transform the expression.

Figure 3

A single logic gate can sometimes be used to replace a more complex logic system.

Figure 4 shows a logic system with inputs A and B and output Q.

Which single logic gate could replace this combinational logic system?

Tick () one box.

Figure 5 shows an operational amplifier circuit designed to function as an oscillator. The circuit produces square waves.

Assume that the operational amplifier is ideal and that it is operating as a comparator in this application.

Explain the combined role of the 33 kΩ resistor and C in this circuit.

State the function of the potential divider formed by the three 100 kΩ resistors.

Deduce the voltage V₊ at the non-inverting input of the operational amplifier when:

• the output voltage V_out is 0.0 V

• the output voltage V_out is 9.0 V.

Figure 6 shows the output from a different oscillator displayed on an oscilloscope screen.

The time-base is 10 μs per division.

Deduce the pulse rate frequency, in kHz, and the duty cycle for this signal.

A photodiode can be used in either photovoltaic mode or photoconductive mode.

State one advantage of using a photodiode in photoconductive mode.

A light-intensity monitor uses a photodiode in reverse bias. The photodiode responds to the irradiance of light incident on it.

Irradiance is a measure of the power of the light incident on unit surface area and is measured in W m⁻².

Figure 7 shows the three stages that make up the light-intensity monitor.

Each stage uses an operational amplifier. The power rails to the operational amplifiers are 0 V and +12 V; these are not shown in Figures 8, 9 or 10.

Figure 8 shows the circuit for stage 1.

When the photodiode detects light within its sensitive range, there is a reverse photocurrent I_D in the circuit. This current depends on the irradiance of the light. The output voltage V_out of the circuit is proportional to I_D. Point X is a virtual earth.

Explain what is meant by a virtual earth.

The output signal V_out from stage 1 is used as the input signal V_in for stage 2.

Figure 9 is an incomplete circuit diagram of stage 2.

Figure 9

Complete Figure 9 to show how an operational amplifier is configured to produce a voltage gain of +34

In your answer:

• label resistors with their values within the range 1 kΩ to 100 kΩ

• show the inverting (−) and non-inverting (+) inputs to the operational amplifier

• do not show the power supplies or power rails for the operational amplifier.

Stage 3 uses an operational amplifier as a comparator to determine whether the irradiance detected by the photodiode in stage 1 exceeds a critical value.

Figure 10 shows the stage 3 circuit diagram.

Figure 11 shows part of the data sheet for the photodiode.

Determine the minimum irradiance that must be detected by the photodiode in stage 1 in order to turn on the LED in stage 3.

When the monitor is tested, the LED turns on fully. However, the LED does not turn off fully when the irradiance falls below the critical value.

Suggest one reason why the LED does not turn off fully. In your answer, refer to a limitation of real operational amplifiers.

State what is meant by a carrier wave within a radio transmission system. Go on to explain how the carrier wave is used in this system.

Figure 12 shows the first stage of a radio receiver.

Explain the role of the LC circuit in the first stage of the receiver.

Two different LC circuits A and B are available for the first stage of a radio receiver tuned to a particular radio station. One of these circuits is to be chosen for the final design of the receiver.

Figure 13 shows the variation of voltage with frequency for both A and B.

The receiver is to detect an AM signal transmitted with a carrier frequency of 200 kHz over a bandwidth of 10 kHz.

Explain whether circuit A or circuit B is more appropriate for this radio receiver.

Your answer should include a discussion of:

• bandwidth

• the ability of the receiver to reject signals from other radio stations.

Figure 14 shows a transmitter T and a receiver R which are not in line of sight. Three transmission pathways from T to R are shown.

Explain, for each of the ground wave, sky wave and space wave:

• the conditions necessary for the pathway to operate

• one advantage and one disadvantage of using that pathway.