Part of a circuit incorporating an operational amplifier (op-amp) is shown in Fig. 9.1.

Question 11

Part of a circuit incorporating an operational amplifier (op-amp) is shown in Fig. 9.1.

Fig. 9.1

(a) A relay is connected to the output of the op-amp circuit so that a lamp may be switched on or off.

(i) Complete Fig. 9.1 to show the relay connected into the circuit. [2]

(ii) State and explain whether the output of the op-amp is positive or negative for the lamp to be switched on. [2]

(b) State the device in Fig. 9.1 that could be used so that the circuit indicates a change in:

(i) the bending of a rod [1]

(ii) the level of daylight to switch on a street light. [1]

[Total: 6]

Reference: Past Exam Paper – June 2019 Paper 42 Q9

Solution:

(a)

(i)

relay coil shown connected between diode and earth

switch shown connected across lamp

(ii) For the diode to conduct, current must flow into the output of the op-amp. So, earth must be at a higher potential than the output of the op-amp {as current flows from higher potential to lower potential}. So, the output of the op-amp must be negative.

(b)

(i) strain gauge

{The resistance of a strain gauge changes with length of the wire/rod.}

(ii) light-dependent resistor