An amplifier incorporating an operational amplifier (op-amp) has three inputs A, B and C, as shown in Fig. 9.1.

Question 4

An amplifier incorporating an operational amplifier (op-amp) has three inputs A, B and C, as shown in Fig. 9.1.

Fig. 9.1

Negative feedback is provided by the resistor RF of resistance 8.0 kΩ.

For each of the inputs A, B and C, the amplifier may be considered as a single input amplifier.

That is, each input is independent of the other two.

When the amplifier is not saturated, the output potential VOUT is given by the expression

VOUT = –(4VA + GVB + VC),

where VA, VB and VC are the input potentials of the inputs A, B and C respectively and G is a constant.

(a) State two effects of negative feedback on an amplifier. [2]

(b) In the expression for the output potential VOUT , the constant G is the gain associated

with input B. Show that the numerical value of G is 2. [1]

(c) The input potentials VA, VB and VC are either zero or 1.0 V.

The magnitudes of some output potentials for different combinations of VA, VB and VC

are shown in Fig. 9.2.

Fig. 9.2

(i) Complete Fig. 9.2 for the three remaining values of VOUT . [1]

(ii) Suggest a use for this circuit. [1]

Reference: Past Exam Paper – November 2009 Paper 42 Q9

Solution:

(a) Choose any 2:

It reduces the gain

It increases the bandwidth

There is less distortion

There is greater stability

(b)

Gain = - R_F / R_I = (-) 8.0 / 4.0 = (-) 2

Numerical value is 2

(c)

(i)

{V_OUT = – (4V_A + GV_B + V_C) = – (4V_A + 2V_B + V_C)

As indicated in the questions, only the magnitudes of V_OUT are to be written in the table.

V_OUT =  – (4(0) + 2(1) + 0) = 2 V

V_OUT =  – (4(1) + 2(1) + 0) = 6 V

V_OUT =  – (4(1) + 2(1) + 1) = 7 V }

2, 6 and 7

(ii) Examples:

Digital-to-analogue converter

Adding / mixing signals with ‘weighting’