(a)[3]
State three characteristics of an ideal operational amplifier (op-amp).
(b)
A circuit using an ideal op-amp is to show whether a door is open or closed. Resistors, each of resistance $R$, are linked to the op-amp inputs, as shown in Fig. 10.1. The switch $S$ is fixed to the door so that, when the door is open, the switch is open. The switch closes when the door is closed.
(b(i))[3]
Explain why the polarity of the op-amp output changes when the switch closes.
(b(ii).1)[1]
A red light-emitting diode (LED) is to show when the door is open. A green LED is to show when the door is closed. On Fig. 10.1, draw symbols for the LEDs to show how they are linked to the output of the op-amp.
(b(ii).2)[1]
Identify the green LED by labelling it G.