A-LevelPhysicsD.C. circuits9702/41 · Oct/Nov 2020 · Q76 Marks

An ideal operational amplifier (op-amp) is to be used in a comparator circuit. Part of the comparator circuit is shown in Fig. 7.1. [Figure 7.1]. Three resistors, each of resistance 1000Ω, and a negative temperature coefficient thermistor are available to complete the circuit. The circuit is to be designed so that, at low temperatures, the output VOUT is –5.0V and at higher temperatures, the output VOUT is to be +5.0V.

(a)On Fig. 7.1, draw the input circuit to the inverting and non-inverting inputs of the op-amp.[4]
(b)(i)State a suitable value for the thermistor resistance when the thermistor is at: low temperature where VOUT is –5.0V[1]
(b)(ii)a higher temperature where VOUT is +5.0V.[1]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[B1] two resistors connected in series between earth and positive of battery and no extra connections
[B1] one resistor and thermistor connected in series between earth and positive of battery and no extra connections
[B1] midpoints of the two potential dividers connected, one each, to the op-amp input terminals
[B1] thermistor in correct place in potential divider circuit (either the upper part of the potential divider leading to V+ or the lower part of the potential divider leading to V-)
[A1] value greater than 1000 Ω
[A1] non-zero value less than 1000 Ω

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