Question

The drain gate capacitance of a junction FET is 2 pF. Assuming common source voltage gain of 20, what is the input capacitance due to Miller effect?
1. 2 pF
2. 40 pF
3. 42 pF
4. 10 pF

Answer 1

Correct Answer - Option 3 : 42 pF

Concept:

The miller effect refers to the increase in equivalent capacitance that occurs when a capacitor is connected from the input to the output of an amplifier with a large, negative gain.

Miller capacitance is given by:

C_Mi = (1 + A_v) C_gs

The output miller capacitance = \({{\rm{C}}_{{\rm{Mo}}}} = {{\rm{C}}_{{\rm{gs}}}}\left( {1 + \frac{1}{{\rm{A}}}} \right)\) 

Calculation:

Given that, A_v = 20

C_gs = 2 pF

CMi = (1 + 20) × 2 = 42 pF