Question

If the internal resistance of the current source is finite, what will happen to the voltage gain. for the following C.S. stage, if K is doubled?

(a) The voltage gain reduces by 1/2

(b) The voltage gain remains the same

(c) The voltage gain increases

(d) The voltage gain decreases

The question was posed to me by my college professor while I was bunking the class.

The question is from MOSFET Amplifier with CS Configuration topic in chapter FET Amplifiers of Analog Circuits

Answer 1

Right option is (b) The voltage gain remains the same

Easiest explanation: The dependent current source has a variable resistance. If K doubles, the magnitude of current provided by the current source doubles, and thus, the total resistance connected to the source of M1 reduces by 2. By using the expression of voltage gain,  \(\frac{-Rd}{(\frac{1}{gm} + Rs)}\), we find that a decrease in Rs leads to an increase in the voltage gain.