Question

If channel length modulation is neglected, what is the voltage gain for the following circuit from source to drain?

(a) gm * Rd

(b) gm * ro

(c) gm * (ro || Rd)

(d) -gm * Rd

The question was posed to me in an interview.

This question is from MOSFET Amplifier with CG Configuration in portion FET Amplifiers of Analog Circuits

Answer 1

Correct answer is (a) gm * Rd

To explain: In absence of channel length modulation, ro=0. The gain is simply gm * Rd. It should be noted that the CG stage doesn’t invert and hence the voltage gain is not -gm * Rd.