Common-gate
In electronics, a common-gate amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a current buffer or voltage amplifier. In this circuit the source terminal of the transistor serves as the input, the drain is the output and the gate is connected to ground, or "common," hence its name. The analogous bipolar junction transistor circuit is the common-base amplifier.
This configuration is used less often than the common source or source follower. It is useful in, for example, CMOS RF receivers, especially when operating near the frequency limitations of the FETs; it is desirable because of the ease of impedance matching and potentially has lower noise. Gray and Meyer provide a general reference for this circuit.
At low frequencies and under small-signal conditions, the circuit in Figure 1 can be represented by that in Figure 2, where the hybrid-pi model for the MOSFET has been employed.
The amplifier characteristics are summarized below in Table 1. The approximate expressions use the assumptions (usually accurate) rO >> RL and gmrO >> 1.
In general the overall voltage/current gain may be substantially less than the open/short circuit gains listed above (depending on the source and load resistances) due to the loading effect.
Taking input and output loading into consideration, the closed circuit voltage gain (that is, the gain with load RL and source with resistance RS both attached) of the common gate can be written as:
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