Question

In an ideal op amp the open-loop gain is 10^3. The op amp is used in a feedback circuit, and the voltages appearing at two of its three signal terminals are measured as v2 = 0V and v3 = 2V where it is assumed that v1 and v2 are input terminals and v3 is the output terminal. The value of the differential (vd) and common-mode (vcm)signal is

(a) Vd = 2 mV and vcm = 1 mv

(b) Vd = 2 mV and vcm = -1 mV

(c) Vd = 2 mV and vcm = 2mV

(d) Vd = 2 mV and vcm = -2mV

This question was posed to me during an online exam.

Asked question is from The Ideal Operational Amplifiers in portion Operational Amplifiers of Electronic Devices & Circuits

Answer 1

Correct answer is (b) Vd = 2 mV and vcm = -1 mV

For explanation I would say: Vc = 0.5(V1 + V2) and

Vd = V2 – V1.