# 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?

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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

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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:

CMi = (1 + Av) Cgs

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

Calculation:

Given that, Av = 20

Cgs = 2 pF

CMi = (1 + 20) × 2 = 42 pF