#### luma

Joined Nov 5, 2015
50
Hello, to everyone i have a doubt about a basic concept of electronic and i hope someone of you can giude me toward the correct answer.

Let's say i have 2 CMOS amplifiers, usually to calculate the transfer function of the whole amplifier I substitute the transistors with their small signal model and I solve the circuit. However if the first amplifier has high gain (lets say 1000), then can we still consider the input signal of the second amplifier "small"? therefore, is it still correct to use the small signal model for the second stage?

Hope someone can help me. Merry christmas.

#### crutschow

Joined Mar 14, 2008
25,653
It depends upon whether the input signal is high enough so that the second stage exhibits a non-linear response to the signal, e.g. the second-stage transconductance value exhibits a significant change over the signal peak-to-to peak voltage.

#### luma

Joined Nov 5, 2015
50
It depends upon whether the input signal is high enough so that the second stage exhibits a non-linear response to the signal, e.g. the second-stage transconductance value exhibits a significant change over the signal peak-to-to peak voltage.
Thank you, but since the fact that the output of the first stage becomes a large signal is a possibility, why electronic book they don't treat this case? I never met a book where they talk about the method to treat a circuit when the input signal is large . and You ?

#### crutschow

Joined Mar 14, 2008
25,653
It's not typically treated in books because it is difficult to do analytically.
The easiest is to do a Spice simulation of the circuit.