Question

How do we calculate the output voltage of a difference amplifier?

(a) VO = -Ad × Vd + AC × VC

(b) VO = -Ad × Vd – AC × VC

(c) VO = Ad × Vd + AC × VC

(d) VO = Ad × Vd – AC × VC

I had been asked this question during an online interview.

My question is from Transistor Difference Amplifiers topic in portion Low Frequency Transistor Amplifier Circuit of Electronic Devices & Circuits

Answer 1

Correct answer is (c) VO = Ad × Vd + AC × VC

To elaborate: The differential gain of a difference amplifier is defined as the gain obtained at the output signal with respect to the difference in the input signals applied. To calculate the output voltage of a difference amplifier we use Ad × Vd + AC × VC. Where, Ad is the differential gain and AC and VC represent the common mode gain.