(a)[3]
Explain why point $P$ is called a virtual earth.
(b)[3]
Derive an expression, in terms of the resistances $R_1$ and $R_2$, for the gain of this amplifier circuit. Explain 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 $V_{OUT}$ is positive.