Question

In the following C.S. stage shown below, what is the transconductance?

(a) \(\frac{1}{2}\)µnCox*(W/L)(V1-Vth)

(b) 3µnCox*(W/L)(V1-Vth)

(c) µnCox*(W/L)(V1-Vth)

(d) 2µnCox*(W/L)(V1-Vth)

The question was asked in an interview for internship.

This intriguing question originated from MOSFET Amplifier with CS Configuration in portion FET Amplifiers of Analog Circuits

Answer 1

Correct choice is (a) \(\frac{1}{2}\)µnCox*(W/L)(V1-Vth)

To elaborate: The transconductance is the ratio of a small change in the output current due to a small change in the input voltage. By differentiating the equation relating the current to the input voltage of a MOSFET with respect to the input voltage, we’ll get \(\frac{1}{2}\)µnCox*(W/L)(V1-Vth).