Question

If Channel Length modulation is neglected, what is the voltage gain from the source to the drain for the CG stage shown below?

(a) gm * Rd * {2Rs / (Rs + 1/gm)}

(b) gm * Rd * {Rs / (Rs + 2/gm)}

(c) gm * Rd * {2Rs / (Rs – 1/gm)}

(d) gm * Rd * {Rs / (Rs + 1/gm)}

The question was asked in an international level competition.

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

Answer 1

The correct option is (d) gm * Rd * {Rs / (Rs + 1/gm)}

Easiest explanation: The voltage gain from the source of M1 to the drain of M1 can be found out from a small signal analysis and it’ll turn out to be gm * Rd. But the input voltage is a result of the potential divider between Rs and 1/gm. Hence, the overall voltage gain is gm * Rd * {Rs/ (Rs + 1/gm)}.