Common gate

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Common gate amplifier

A common-gate amplifier is one of the possible configurations of FET electronic amplifier. It is normally used to convert low impedance to high impedance. This configuration is used less often than the common source or source follower, but is useful in CMOS RF receivers, especially when pushing the frequency limitations of the FETs, because of the ease of impedance matching and potentially lower noise.

[edit] Characteristics

(The parallel lines indicate components in parallel.)

Inherent voltage gain:

${V_\mathrm{out} \over V_\mathrm{in}} = g_m (R_\mathrm{D} \| R_\mathrm{load})\,$

Input resistance:

$r_\mathrm{in} = R_\mathrm{S} \left\| {1 \over g_m}\right.$

Current gain:

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

Output resistance:

$r_\mathrm{out} = R_\mathrm{D}\,$

The variables not listed in the schematic are:

g_m is the transconductance in siemens

[edit] DC Coupling

This circuit can also be DC coupled in which case wires replace the capacitors C_in and C_out and R_s can be removed. If the input is DC coupled then the circuit is typically part of a cascode.

[edit] See also

Transistor amplifiers
Bipolar junction transistor: Common emitter • Common collector • Common base Field effect transistor: Common source • Common drain • Common gate Multiple transistors: Darlington transistor • Cascode • Parallel circuit