A-LevelPhysicsD.C. circuits9702/41 · Oct/Nov 2021 · Q79 Marks

State two properties of an ideal operational amplifier (op-amp).

(a)State two properties of an ideal operational amplifier (op-amp).[2]
(b)[Figure 7.1] shows a circuit that includes an ideal op-amp and two identical resistors R. State the names of components X and Y.[1]
(c)(i)Explain why the op-amp in [Figure 7.1] has only two possible output states.[2]
(c)(ii)State the name of the type of op-amp circuit in which the op-amp behaves as in (c)(i).[1]
(c)(iii)Describe the environmental condition under which the lamp L in [Figure 7.1] will light.[2]
(c)(iv)Suggest the purpose of the variable resistor V in the circuit.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

[B1] infinite (open-loop) gain
[B1] infinite slew rate
[B1] infinite input impedance
[B1] zero output impedance
[B1] infinite bandwidth
[B1] X: thermistor and Y: relay
[B1] (any) difference in voltage at the inputs causes output to saturate (because gain is very large)
[B1] saturates positively if V⁺ > V⁻ and saturates negatively if V⁺ < V⁻
[B1] comparator
[M1] temperature
[A1] above a particular value
[B1] to adjust the temperature (at which the lamp illuminates/extinguishes)

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