AQA A Level Physics Paper 3 Section B: Electronics June 2018 question paper 7408/3BE

Two logic inputs, A and B, feed into the logic circuit shown in Figure 1. The logic output from the circuit is Q.

Deduce the Boolean expression for the output of this logic circuit in terms of inputs A and B. Include all the logic operations that take place between the inputs and the output.

The truth table shows some of the logic states for the logic gates in Figure 1.

Complete the truth table.

Figure 2 shows a different logic circuit that produces the same logic output as that of Figure 1.

A manufacturer wants to produce a system that uses this logic function, but is undecided as to which circuit to use.

Suggest, giving reasons, two benefits of using the logic circuit in Figure 2 compared to the logic circuit in Figure 1.

State the single logic gate that would perform the same logic function as the circuits shown in Figure 1 and Figure 2.

An ultrasound sensor produces an output that needs to be amplified to 3.0 V The amplifier used has a voltage gain of 40

Calculate the input voltage V in to the amplifier from the sensor.

An operational amplifier in non-inverting mode is used to amplify the output of the sensor. The partially completed circuit diagram is shown in Figure 3.

Complete the circuit diagram in Figure 3 by adding and labelling two resistors, R in and R f, so that the operational amplifier is correctly configured in its non-inverting mode. The power lines should not be shown in the completed diagram.

Determine, using resistors selected from the list below, how the voltage gain of 40 can be achieved by the non-inverting amplifier of Figure 3.

1 kΩ 3.6 kΩ 10 kΩ 39 kΩ 150 kΩ

The ultrasound frequency detected by the sensor is 50 kHz For this operational amplifier

gain × bandwidth = 1.0 MHz

Discuss whether this operational amplifier is suitable for amplifying the sensor’s output voltage.

Figure 4 shows a block (subsystem) diagram for a radio communication system.

State the letter representing the subsystem in which you might find an induced emf

State the letter representing the subsystem where the audio and radio waves are

The signal strength at stage D must be amplified.

Explain why the signal strength at stage D is weak.

Figure 5 shows graphs of a radio carrier signal and a modulating sine wave.

Complete the graph in Figure 5 to represent the combined amplitude modulated (AM) signal.

Approximately 20 radio stations use amplitude modulation (AM) to broadcast to people living in the London area. Another 35 AM stations broadcast to people outside the London area. However, these broadcasts can still be received in London.

The allocated frequency spectrum for all these broadcasts is in the range 540 kHz to 1600 kHz

Suggest whether all these stations can broadcast hi-fi music using the full audio frequency of 20 kHz

Figure 6 shows an astable circuit based on a NOT logic gate. The symbol in the centre of the logic gate means that the output V o changes at two different input values of V c depending on whether the input voltage is rising or falling.

The pulse repetition frequency (PRF) for this particular circuit is given by:

Calculate the PRF in kHz

The supply voltage to the NOT gate is V s

• When V_c increases and reaches the upper switching threshold (UST), the output of the NOT gate will switch from V s to 0 V

• When V_c decreases and reaches the lower switching threshold (LST), the output of the NOT gate will switch from 0 V to V s

The graph in Figure 7 shows V c constantly changing as the capacitor charges and discharges.

Draw on Figure 7 the output voltage V o for the astable circuit.

The circuit in Figure 6 can be modified by the addition of a resistor to vary the PRF .

The astable is to be modified so that it produces a frequency 4 times that of the original.

Calculate the value of the resistor that should be added to the circuit and explain where in the circuit this additional resistor should be placed.

In another astable, two resistors (R 1 and R 2) and a capacitor C form a timing chain to control the mark and space times for a square wave produced at the output of the integrated circuit (IC) shown in Figure 8.

The charging time for the capacitor C is: t C = 0.7 × (R 1 + R 2) × C The discharging time for the capacitor C is: t D = 0.7 × R 2 × C

Calculate, in kΩ, values for R 1 and R 2 needed to produce a 5 kHz signal with 75% duty cycle given that the capacitor C has a value of 10 nF

The output of the IC in Figure 8 is 5 V during the charging period and 0 V during the discharging period.

Draw on Figure 9 the wave pattern that represents this signal.

In a recording studio the output from a microphone is an analogue signal. The equipment in the studio converts this analogue signal into a digital signal before storing it.

Discuss aspects of the analogue-to-digital conversion in this context.

In your answer you should include:

• what is meant by quantisation

• factors that affect the quality of the digital version of the analogue signal

• the advantages and disadvantages of digitising the analogue signal.

You may use diagrams to help make clear aspects of your answer.