the source follower

The source follower, also called the common-drain stage, is the MOSFET buffer: it does not amplify the voltage, but it amplifies the circuit's ability to drive a load. The input goes to the gate, the output is taken from the source (across a source resistor to ground), and the drain ties to the supply. The source voltage simply follows the gate, sitting one gate-to-source drop below it, so the output is a copy of the input that the next stage can lean on heavily.

Its voltage gain is just under one (typically 0.8 to 0.99), so it gives no voltage boost. What it gives is impedance transformation: a very high input resistance (the insulated gate) and a low output resistance of roughly 1/gm, often tens or low hundreds of ohms. This lets a delicate, high-impedance signal source drive a heavy or capacitive load without sagging, which is exactly what a buffer is for. It is the FET twin of the BJT emitter follower.

Use a source follower whenever you need to take a high-impedance node (a sensor, a filter output, a previous gain stage) and drive something demanding (a long cable, a low-resistance load, an ADC input) without loading it down. The honest caveats are that the output sits a threshold-plus drop below the input (it is not at the same DC level), and the modest, current-dependent gain means it is a buffer, not an amplifier: do not expect it to make a signal bigger.