Circuit intuitions: Example 2. Feedback CS NMOS

Let us analyze the effects of feedback in common source NMOS. Refer the below figure. (a) Shows the resistance looking into drain node when a voltage gain amplifier is inserted between the drain and gate of NMOS transistor. This output voltage is amplified and added to the input voltage. This is feedback circuit. 

If the input resistance of the voltage amplifier is infinite, then the output resistance is given by Req. 

Refer figure (b), the feedback gain is A = 1, the effective circuit is popular diode-connected NMOS where drain and gate are tied together.
Now  refer figure (c) when there is no feedback i.e., A = 0 the circuit is simply NMOS and the effective drain resistance is ro of NMOS.

• When A is greater than unity, it looks like g_m is increased by an amount A.G_m =A g_m  This is called g_{m -}boosting technique.

• When A is negative Req is negative.  Using this technique we can create negative resistances.

We will see the applications of the above two techniques in the future posts.