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Common drain

A common-drain (commonly called a source follower) amplifier is one of the common configurations of FET electronic amplifier and is similar in operation to a BJT emitter follower (common collector circuit) in that the source voltage closely follows the voltage at the gate terminal.

Characteristics

(The parallel lines indicate components in parallel)

Inherent voltage gain:

$g_m (R_S \| R_{load}) \over 1 + g_m (R_S \| R_L)$

Input resistance:

$R_1 \| R_2,$

Current gain:

$A_{vm} {r_{in} \over R_{load}}$

Output resistance:

$R_S \| {1 \over g_m}.$

Transistor amplifiers

Common emitter

Common collector

Common drain

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