the common-gate amplifier

The common-gate amplifier is the third and least common MOSFET configuration, the mirror of the BJT common-base stage. Here the gate is held at a fixed voltage (the common reference), the input signal is injected at the source, and the output is taken from the drain. Pushing current into the source modulates the channel and produces an amplified, non-inverting voltage at the drain. It is the odd one out: you drive the source, not the gate.

Its defining trait is a low input resistance (roughly 1/gm, since you are pushing against the source), which is unusual and occasionally exactly what you want, for example to terminate a transmission line or to accept a current signal. Its voltage gain is similar in magnitude to the common-source stage, about gm times the drain resistance, but the output is in phase with the input (no inversion). The standout virtue is excellent high-frequency behaviour: it sidesteps the Miller effect that limits the common-source stage, so it shines at radio frequencies.

You rarely meet a common-gate stage on its own; its real home is as the top half of a cascode, stacked above a common-source stage. In that pairing the common-source device gives the gain while the common-gate device shields it from the Miller effect, giving high gain AND high bandwidth together. As a beginner, know it mainly as the low-input-resistance, non-inverting, high-frequency member of the FET amplifier trio.