AQA A Level Physics Paper 3 Section B: Electronics June 2023 question paper 7408/3BE

Figure 1 shows the input and output pins for a 4-bit binary counter. The output pin for the least significant bit is A.

Complete Figure 1 by adding a single logic gate to the binary counter so that the circuit functions as a modulo-6 counter.

Figure 2 shows three outputs of the modulo-6 counter connected to a logic sub-system that controls a 7-segment display. The decimal point on the display is not shown. The whole system shown in Figure 2 is to be used as an electronic dice.

Segments in the 7-segment display are turned on or off by the logic sub-system to display the decimal numbers 1 to 6 in sequence. A segment in the display turns on when the logic output with the same letter as the segment is at logic 1.

Table 1 shows how the values of CBA control the logic level applied to each of the segments a to g during the counting cycle.

One of the logic outputs a to g is controlled by a single NOT gate. This gate uses one of the inputs A, B or C.

State the input that is used and the segment that this NOT gate controls.

X represents one of the logic outputs. The Boolean expression for this output is:

State which of the logic outputs a to g is being controlled by this function.

Y represents another of the logic outputs. The Boolean expression for this output is:

Complete Figure 3 to show the combination of logic gates needed to represent the expression. You should only use logic gates that represent the individual functions shown in the expression.

Figure 3

Figure 4 shows a strain gauge that consists of a folded metal wire glued to a piece of insulating material.

The resistance of the metal wire:

• increases when the gauge is stretched by forces applied as shown

• changes when there is a change in temperature.

Figure 5 shows a circuit that contains two identical strain gauges A and B. The resistance of each strain gauge is 120.00 Ω when the strain is zero.

Assume that the voltmeter is ideal.

The circuit is used to measure changes in strain in a metal beam. A is glued firmly to the metal beam. B is placed on the beam near A but is not glued to the beam.

Initially, with the gauges under zero strain, R₂ is adjusted to produce 0 V on the voltmeter. When the beam bends, A stretches and B remains unstretched. The resistance of A increases to 120.06 Ω.

Calculate, in mV, the new reading on the voltmeter.

Explain the advantage of using strain gauge B rather than a 120.00 Ω resistor in the circuit.

The voltages at X and Y in Figure 5 are now used as inputs to an operational amplifier. The operational amplifier is configured as a difference amplifier with a gain of +5

Complete Figure 6 to show the difference amplifier circuit. Any resistors used must be labelled with their values. These values must be in the range 10 kΩ–10 MΩ. Do not show the supply rails.

Figure 6

Figure 7 shows a block diagram for part of a communication system that transmits control information from Earth to a satellite.

Syncom 3 was the first geostationary communication satellite. The satellite was used to broadcast television signals of the 1964 Olympic Games from Tokyo in Japan to Los Angeles in the USA.

State what is required so that de-sensing does not occur in a radio receiver such as that on Syncom 3.

Commentators in Los Angeles interviewed athletes in Tokyo using Syncom 3.

The commentators found that there was a significant time delay between the end of each question and the arrival of its reply.

This time delay was made up of:

• the total time that the signals spent travelling

• the total processing time.

The distance of the geostationary satellite from both Tokyo and Los Angeles was 40 000 km.

The time delay was 900 ms.

Calculate, in ms, the total processing time.

The television signals received in the USA were relayed to the UK using a satellite in low Earth orbit. The availability of live coverage was different for viewers in the USA and viewers in the UK.

Explain why.

For the 2020 Olympics, digital TV signals between Tokyo and Los Angeles used a fibre-optic cable. The cable lies on the seabed.

Explain why a fibre-optic cable rather than a satellite link is now preferred for intercontinental communication.

A particular operational amplifier has the following properties:

• open-loop gain = 1 × 10⁵

• break frequency (cutoff frequency) = 10 Hz

• gain × bandwidth = 1 MHz.

Draw, on Figure 8, the variation of gain with frequency for this operational amplifier in open-loop mode.

Figure 9 shows the operational amplifier being used in an inverting amplifier circuit.

Annotate Figure 9 with:

• an X to indicate a point that acts as a virtual earth

• arrows to show the direction of current I in both resistors when the

input voltage V_in is negative.

The value of R_f in the feedback loop in Figure 9 is increased.

State and explain the effect of this change on the bandwidth of the inverting amplifier.

Figure 10 shows a circuit that has an input signal which is a noisy square wave. The circuit is used to remove the noise and switch a motor M on and off.

Explain the functions of components A, B and C.

For each of these components you should:

• explain its role in the circuit

• identify the characteristic properties that make the component suitable for this role.