AQA A Level Physics Paper 3 Section B: Electronics November 2021 question paper 7408/3BE

Figure 1 shows a simplified structure of an N-channel enhancement mode MOSFET.

State the name of the part shown in this MOSFET structure that causes the input resistance to be very large.

Which terminal of the MOSFET is connected directly to 0 V when it is used as a simple switch?

Tick () one box.

Figure 2 shows how the drain–source current I_DS of the MOSFET varies with drain–source voltage V_DS for a range of gate–source voltages V_GS.

The MOSFET is used as a simple switch in a filament lamp circuit. The circuit uses power rails of 12 V and 0 V. The resistance of the lamp is 154 Ω when operating at its full power of 0.65 W.

Deduce the minimum value of V_GS needed for the lamp to operate at full power.

Figure 3 shows an individual MOSFET. The drain–source leakage current I_DSS for this MOSFET is about 10 nA.

Figure 4 shows a microchip where millions of MOSFETs are combined to enable complex processes to be carried out on one chip.

A mobile phone has a central processing unit (CPU) which uses a microchip similar to the one in Figure 4.

Table 1 shows the technical specification for the mobile phone.

A fully charged battery with a capacity of 1 A h allows 3600 C of charge to flow through it before it is fully discharged.

The I_DSS value for each MOSFET used in the mobile phone CPU must be different from that measured in the individual MOSFET shown in Figure 3.

Discuss, using the data provided, the reason for this difference.

A burglar-alarm system in a house sounds an alarm during the hours of darkness when one of the following conditions is met:

• the door is opened

• the window is opened

• both the door and the window are opened.

Figure 5 shows the main burglar-alarm subsystems and the logic status for the inputs and output.

Table 2 is a partially completed truth table for the logic subsystem.

Complete Table 2.

Figure 6 shows the symbol of the push-to-make switch used in the door detector. When the door is closed, the switch button is pushed down onto the contacts. It automatically releases when the door opens.

Complete Figure 7 to show how this switch, together with a 10 kΩ resistor, can be connected to create the door detector circuit in Figure 5.

Label the output of the circuit with an X.

Figure 8 shows a logic circuit for a different alarm system.

Write the Boolean algebra expression for Q in terms of inputs A, B and C. In your answer use only AND and NOR operators.

Figure 9 shows a logic circuit that has the same function as the circuit in Figure 8. Only one type of gate is used in the circuit in Figure 9.

State the logic function performed by gate 4.

Microchips containing two-input logic gates are mass-produced. Each microchip contains four identical logic gates.

A manufacturer of the logic circuit used in the burglar alarm chooses to make the circuit in Figure 9 rather than that in Figure 8.

Suggest why.

Figure 10 shows the circuit for an infrared detector using a photodiode and an operational amplifier. In this application the operational amplifier uses a feedback resistor to give a voltage signal when the current in the photodiode changes.

State the mode in which the photodiode is being used in Figure 10.

In the circuit shown in Figure 10, there is a current in the photodiode even when there is no light incident on it. This current is called the dark current.

In an optical communication system, the dark current needs to be very small in comparison to the photodiode current.

Explain why.

The responsivity R_λ of a silicon photodiode is a measure of its sensitivity to light at a given wavelength λ.

R_λ is defined as:

where I_p is the current in the photodiode and P is the incident light power at the given wavelength.

Figure 11 shows the spectral response graph for this photodiode.

Monochromatic radiation of wavelength 850 nm and power 4.0 µW is incident on the photodiode in Figure 10.

Calculate the output voltage of the detector circuit.

The output from the detector circuit in Figure 10 needs to be amplified by a factor of +4

Complete Figure 12 to show the amplifier circuit required.

In your completed circuit you should:

• label the input point as V_in

• label your diagram with the values of resistance for any resistors used in your circuit. Any resistance values must lie within the range 1 kΩ to 100 kΩ.

Do not show the power supplies to the operational amplifier.

Figure 12

A Johnson decade counter uses a Johnson counter together with decoding logic. This arrangement produces a single logic 1 at a series of outputs Q₀–Q₉ in a continuous sequence.

Describe one functional difference and one functional similarity between how a Johnson decade counter and a BCD counter output their counts.

functional difference

functional similarity

An astable oscillator produces a continuous train of pulses. Figure 13 shows the display of the pulses on an oscilloscope.

The oscilloscope settings are:

voltage gain = 2 V / division time-base = 50 µs / division.

Determine the duty cycle and frequency of the signal.

The astable is adjusted to produce a 600 Hz test signal.

This signal is applied to the clock input of the BCD counter and to the clock input of the Johnson decade counter.

The outputs of the BCD counter are Q₀, Q₁, Q₂ and Q₃ where Q₀ is the least significant part of the output. The outputs of the Johnson decade counter are Q₀, Q₁, Q₂…Q₉.

Determine the frequency of the pulses available at Q₂ for each counter.

Table 3 shows some communication applications that transmit using different regions of the electromagnetic spectrum.

Explain why each transmission takes the pathway it does from the transmitter to the receiver.

For each of the spectrum regions, you should:

• indicate a frequency range

• refer to the properties of the wave

• name the pathway and outline its properties.

You may use diagrams to help explain your answer.

Space for diagrams