The inverting-amplifier circuit that uses an ideal operational amplifier (op-amp) is illustrated in Fig. 8.1.
(a)[3]
Explain why point $P$ is referred to as a virtual earth.
(b)[3]
Derive an expression for the gain of the amplifier circuit in terms of the resistances $R_1$ and $R_2$. Show your working.
(c)[3]
On Fig. 8.2, indicate how the relay can be linked to the amplifier output so that it operates only when $V_{\text{OUT}}$ is positive.
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