Explain working of common drain amplifie, Electrical Engineering

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Q. Explain working of Common drain amplifie?

When an ac signal drives the gate of JFET, it produces an ac drain current. It flows through Rs and produces an ac output voltage that is approximately equal to the input voltage and is in phase with it.

1895_COMMON DRAIN AMPLIFIER.png

fig.(a)  COMMON DRAIN AMPLIFIER

In the figure, R3 = Rs.

From the figure:

Vin = Vgs+gmVgsRs

Vin = (1+gmRs) Vgs

Vout = gmVgsRs

Vout/Vin = gmVgsRs/ (1+gmRs) Vgs = Rs/(1/gm)+Rs

Rs> = 1/g

Vout = Vin Rs/(Rs+(1/gm))

Zin depends on the type of biasing circuit used. In case of voltage divider biasing circuit, Zin = R1 and R2 in parallel.

The common drain amplifier with high input impedance and near-unity voltage gain is used as a buffer amplifier. The common drain amplifier is also known as "SOURCE FOLLOWER".


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