AQA A Level Physics Paper 3 Section B: Electronics June 2019 question paper 7408/3BE

Figure 1 shows the basic layout for a Johnson decade counter. The main input is the clock (CK). The main outputs are shown as Q 0 to Q 9.

Figure 2 shows part of the timing diagram for a Johnson decade counter. This timing diagram shows the output logic states against time. The counter is reset to make Q 0 = 1 and then the first two pulses are applied.

Complete Figure 2 to show the logic states of Q 0, Q 1 and Q 2.

A student sets up the counter to make the traffic light sequence:

red → red + amber → green → amber

The sequence then repeats.

Figure 3 shows a partially completed diagram for producing this sequence.

Draw an OR gate and connections on Figure 3 so that the LEDs go through the complete sequence.

Figure 3

State two factors that determine the ON time for the green LED shown in Figure 3.

The potential difference across the green LED is 2.1 V when it is lit. The current through it should not exceed 9 mA.

All logic gate outputs are: logic low = 0 V logic high = 9 V.

The student suggests that a resistor of resistance 720 Ω and a tolerance of ±5% should be used for R.

Deduce whether the student’s suggestion would be suitable.

Figure 4 shows an operational amplifier circuit used in an audio mixing desk.

The power supply for the amplifier is –12 V and +12 V but this is not shown in Figure 4.

What is the operational amplifier configuration shown in Figure 4 ? Tick (✓) one box.

The circuit shown in Figure 4 is tested by making the following connections:

• input A is connected to an audio signal of amplitude 150 mV

• input B is connected to 0 V.

Calculate the amplitude of the output voltage.

A microphone converts a sound wave into the voltage signal labelled signal 1 in Figure 5. At the same time the microphone produces a second signal, labelled signal 2. Signal 2 is the inversion of signal 1. These two signals travel along two separate wires in the same cable.

Figure 6 shows some electrical noise that has been picked up and added to the signals as they travel through the cable from the microphone to the operational amplifier circuit in Figure 4.

The connections made in question 02.2 are removed. Signal 1 is connected to input A and signal 2 is connected to input B.

Explain how the operational amplifier circuit affects the noise and strength of the output signal.

Figure 7 shows a system to monitor a tank filling with liquid in which a magnet is mounted on a float.

The Hall effect sensor produces an output voltage V. V depends on the distance d between the sensor and the magnet.

When V reaches a certain value, the flow of liquid to the tank is switched off.

The magnet may be arranged with either the north (N) or south (S) pole facing the sensor.

Figure 8 shows how the magnitude of V varies with d for the two possible

arrangements of the magnet.

Compare the advantages of the two arrangements for monitoring the movement of the magnet towards the Hall effect sensor.

In your answer you should compare:

• the sensitivity of the system

• the range of d over which the system is useful.

You may ignore any effect from the Earth’s magnetic field.

Figure 9 shows a Hall effect sensor being used as a tachometer to monitor the rotational speed of the drive shaft of an electric motor.

The output of the Hall effect sensor is connected to an oscilloscope. When the vane is between the magnet and the Hall effect sensor, the output of the Hall effect sensor is low.

The trace produced on the oscilloscope is shown in Figure 10.

The time-base on the oscilloscope is set to 5 ms / div.

Calculate the number of complete revolutions of the drive shaft in one second.

A silicon-based 5.1 V Zener diode requires a minimum operating current I of 5.0 mA to maintain its Zener voltage V_z.

Draw on Figure 11 the general IV characteristic for this diode.

Figure 12 shows a circuit that uses a 5.1 V Zener diode. The circuit causes the output W of the operational amplifier to change at a particular light intensity.

State the function of the Zener diode in this circuit.

Deduce whether a 100 Ω, 0.13 W resistor is suitable for R in Figure 12.

The circuit in Figure 12 is rebuilt and the position of R is swapped with the position of the Zener diode.

Explain how this affects the light intensity at which W changes.

The output W from the operational amplifier shown in Figure 12 becomes one of three inputs to the combinational logic circuit shown in Figure 13.

Write the Boolean algebra expression for the output Q in terms of W, X and Y based on the logic gates shown in Figure 13.

Q =

Output Q from Figure 13 becomes the input to the final part of the circuit shown in Figure 14.

The circuit uses a MOSFET to activate a relay.

State one property that makes the MOSFET suitable for interfacing with logic gates.

A telephone company transmits 15 speech channels across a single transmission link. The analogue information is first subjected to pulse code modulation (PCM) before transmission using time division multiplexing (TDM). The transmission system incorporates regenerators which remove noise from the signal.

Explain the basic principles of:

• pulse code modulation (PCM)

• time division multiplexing (TDM)

• a regenerator.