Question

Assume µnCox = 100 µA/V^2 and supply current is 5mA, what should be the aspect ratio so that a 50 Ω load can be used to give a voltage gain of .25 in C.D. configuration?

(a) 32.6

(b) 50

(c) 40

(d) 41

The question was asked in an interview for job.

I'm obligated to ask this question of MOSFET Amplifier with CD Configuration in division FET Amplifiers of Analog Circuits

Answer 1

The correct choice is (a) 32.6

To explain I would say: The voltage gain is given by RL/(1/gm + RL) where RL is the 50 Ω load. Now, we see that if the voltage gain is .25, gm is 1/175. Now, gm is \(\sqrt{(2* µ_nC_{ox} * (W/L) * Id)}\) where (W/L) is the aspect ratio, Id is the drain current. We have all the values and the aspect ratio becomes 32.6.