An ideal operational amplifier (op-amp) is to be used as a comparator circuit. Fig. 7.1 shows part of the comparator circuit. To finish the circuit, three resistors, each with resistance $1000\,\Omega$, and a negative temperature coefficient thermistor are available. The circuit should be arranged so that, at low temperatures, the output $V_{\text{OUT}}$ is $-5.0\,\text{V}$, whereas at higher temperatures, the output $V_{\text{OUT}}$ is $+5.0\,\text{V}$.
(a)[4]
On Fig. 7.1, sketch the input circuit for the inverting and non-inverting inputs of the op-amp.
(b(i))[1]
State a suitable thermistor resistance at low temperature when $V_{\text{OUT}} = -5.0\,\text{V}$.
(b(ii))[1]
State a suitable thermistor resistance at a higher temperature when $V_{\text{OUT}} = +5.0\,\text{V}$.
Worked solution & mark scheme
This 6-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Accurate series potential dividers using the resistor and thermistor” …