Common gate

In this circuit, the source terminal of the transistor serves as the input, the drain is the output, and the gate is connected to some DC biasing voltage (i.e. an AC ground), or "common," hence its name.

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.

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: which has the simple limiting forms depending upon whether gmRS is much larger or much smaller than one.

The current gain is unity, so the same current is delivered to the output load RL, producing by Ohm's law an output voltage vout = vThévRL / RS, that is, the first form of the voltage gain above.

In the second case RS << 1/gm and the Thévenin representation of the source is useful, producing the second form for the gain, typical of voltage amplifiers.

Figure 1: Basic N-channel common-gate circuit (neglecting biasing details); current source I D represents an active load ; signal is applied at node V in and output is taken from node V out ; output can be current or voltage
Figure 2: Small-signal low-frequency hybrid-pi model for amplifier driven by a Norton signal source
Figure 3: Hybrid pi model with test source i x at output to find output resistance