What is Miller effect and how is effecting in the cascode transistor

[narayani]

In the below screen shot, upper transistor is connected in cascode with down transistor to minimize the miller effect on down transistor and give higher gain.

What is this Miller effect, how Miller effect is effecting on the down transistor, what is the significance of the Miller effect.

 

[chuckey]

Think of a capacitor connect between the gate and the drain. If the voltage changes by vi on the gate the voltage changes on the drain by -G(gain) X vi. So the voltage across the capacitor has changed by (G + !) X vi. So looking in from the gate the capacitance has increased by G+1. So as the gain is changed the input capacitance changes.
In the cascode amp, the voltage on the drain changes very little as its input impedance is 1/Gm (~200 ohms?), so the input capacity of the first transistors gate will be twice Cg-d. The AC component now modifies the G-D voltage of the top FET, so it amplifies as usual. It will have no Miller effect as the gate is decoupled to earth, so the Cg-d, just appears across the load resistor.
Frank