Question

For an ideal current source, what is the voltage gain of the following circuit?

(a) gm1 * rd1 * gm2 * 2Rd2

(b) – gm1 * rd1 * gm2 * Rd2

(c) 2gm1 * rd1 * gm2 * Rd2

(d) – gm1 * rd1 * gm2 * Rd2

I got this question during an interview.

Origin of the question is MOSFET Amplifier with CG Configuration in portion FET Amplifiers of Analog Circuits

Answer 1

Right choice is (d) – gm1 * rd1 * gm2 * Rd2

The best explanation: The voltage gain for the C.G. stage is simply gm * Rd1. The voltage gain of the next stage is – gm * Rd2. Note that, the ideal current source exhibits infinite output impedance and doesn’t affect but only increases the linearity of operation.