Question

If g is the transconductance and r is the resistance due to channel length modulation – what is the total voltage gain if only M1 has channel length modulation?

(a) R / ((1/g || r ) + R) * g *R * R / (1/g + R)

(b) R / ((1/g || r ) + R) * g *R * R * g

(c) R / ((1/g || r )) * g *R * R / (1/g + R)

(d) R / ((1/g ) + R) * g *R * R / (1/g + R)

I got this question in semester exam.

I need to ask this question from MOSFET Amplifier with CD Configuration topic in portion FET Amplifiers of Analog Circuits

Answer 1

The correct option is (a) R / ((1/g || r ) + R) * g *R * R / (1/g + R)

To explain I would say: This is a cascade of a follower stage follower by a CS stage which precedes another follower stage. The gain due to each stage gets multiplied until we reach the output. For the first stage, the gain is R/ ((1/g || r)+ R) since M1 has channel length modulation. The second stage has a gain of g * R while the final stage has a gain of R/ (1/g + R). After multiplying the gains, we get R/ ((1/g || r )+ R) * g *R * R/ (1/g + R).