A-LevelPhysicsD.C. circuits9702/42 · May/June 2020 · Q811 Marks
An ideal operational amplifier (op-amp) is connected to a load resistor. The op-amp is assumed to have infinite bandwidth and zero output resistance. A student designs the circuit shown in Fig. 8.1 in order to indicate changes in temperature of the thermistor T. [Figure 8.1]
- (a)(i)State: what is meant by infinite bandwidth[1]
- (a)(ii)the effect, if any, on the output voltage of increasing the load resistance.[1]
- (b)(i)Explain why point P is known as a virtual earth.[3]
- (b)(ii)Calculate the potential at point Q.[2]
- (b)(iii)At a temperature of 13°C, the resistance of the thermistor T is 230 kΩ. Show that the potential difference measured with the voltmeter is 0.88 V.[2]
- (c)The resistance of the thermistor T in (b) decreases as its temperature rises. Explain the effect of this change in temperature on the potential difference measured with the voltmeter.[2]
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Mark scheme
Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.
- [1] constant gain for all frequencies
- [1] unchanged
- [1] (open loop) gain of op-amp is infinite
- [1] feedback loop ensures V⁺ ≈ V⁻
- [1] any difference between V⁺ and V⁻ results in saturated output
- [1] non-inverting input is 0 V so inverting input also at 0 V
- [1] input = (40 × 1.5) / (40 + 110)
- [1] = 0.40 V
- [1] gain = (-) (100 + 230)/150
- [1] feedback current = 0.40 / (150 × 10³)
- [1] p.d. = [(100 + 230) / 150] × 0.40
- [1] = 0.88 V
- [1] (magnitude of) gain decreases
- [1] voltmeter reading decreases
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