A-LevelPhysicsD.C. circuits9702/42 · May/June 2021 · Q79 Marks

Two properties of an ideal operational amplifier (op-amp) are infinite input impedance and infinite bandwidth.

(a)(i)State what is meant by: infinite input impedance[1]
(a)(ii)infinite bandwidth.[1]
(b)(i)Show that the potential at point X is 0.40 V.[1]
(b)(ii)The thermistor has a resistance of 360 kΩ at a particular temperature. For this temperature of the thermistor, calculate the magnitude of the reading on the voltmeter.[3]
(b)(iii)The temperature of the thermistor increases. State and explain the effect of this change on the magnitude of the reading on the voltmeter.[2]
(b)(iv)Explain why the amplifier circuit will no longer indicate temperature changes when the magnitude of the gain of the circuit is greater than 12.5.[1]

Mark scheme

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

[1] no current enters/leaves the input
[1] gain is the same for all frequencies
[1] V_IN = 1.5 × 400 / (400 + 1100) = 0.40 V OR V_IN = 1.5 - (1.5 × 1100 / 1500) = 0.40 V OR (1.5 – V_IN) / 1100 = V_IN / 400 so V_IN = 0.40 V
[1] gain = (-) R_f / R_i
[1] V_OUT / 0.40 = (360 + 100) / 96
[1] V_OUT = 1.9 V
[1] resistance of thermistor decreases
[1] (magnitude of) gain decreases so reading decreases
[1] (at gain 12.5) V_OUT is 5.0 V, so (above gain 12.5) output becomes saturated

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