A basic but important question~

[joehwang]

Hi,

I got a very basic question, could someone help me to make sure the answer?

I have checked some useful textbooks(EX:razavi), however, I still feel not enough.
1. what is the difinition for "small signal"
2. What is the benefit for using the small signal analysis? Is there any condition for using the small signal analysis?
3. How small the signal can called "small signal"? Is there any definition for the boundaay?

thanks!

 

[lladnar23]

"small signal" is anything that is small enough for the circuit to behave linearly. How small that is depends upon how linear your system is. If you have a nonlinear system (such as a diode -- exp(-V/vd) ), you need to consider much smaller voltages than if you had a very linear system (e.g. a resistor).

A common rule of thumb for square-law MOSFETs is to make sure your input voltage is much less than 2(VGS-VT).

 

[zopeon]

Small signal is that level of signal which does not alter the bias condition of ur amplifier. So the benefit is u need not check for the state of mosfet ( Lin,Sat etc) for every input and thus just deal with the ac signal ( grnd DC sources )