Question

If channel length modulation is neglected, what is the voltage gain for the following circuit?

(a) – {gm1 * Rd * Rd / (1/gm2 + Rs)}

(b) – (gm1 * 2Rd * Rd / Rs)

(c) – {gm1 * Rd * Rd / (1/gm2 + Rs)}

(d) – {gm1 * Rd * Rd / (1/gm2 + 2Rs)}

I got this question in an international level competition.

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

Answer 1

Correct option is (a) – {gm1 * Rd * Rd / (1/gm2 + Rs)}

The best I can explain: This is a cascade of a CG stage preceding a CS stage. For the CG stage, the voltage gain is gm * Rd. But after this stage, the signal gets amplified by a degenerated CS stage with a factor of -Rd / (1/gm2 + Rs). Hence, the overall voltage gain is the product of both the factors i.e. – {gm1 * Rd * Rd / (1/gm2 + Rs)}. Note that the input impedance looking into M2 is infinite ad hence the voltage gain of the C.G. stage remains unaffected by the C.S. stage (for low frequency operations only).