the common-source amplifier
The common-source amplifier is the MOSFET's main voltage-amplifying configuration, the direct counterpart of the BJT's common-emitter stage. The input signal goes to the gate, the output comes from the drain (through a drain resistor to the supply), and the source is the common reference shared by input and output. A small wiggle on the gate produces a much larger, inverted wiggle at the drain: it amplifies, and it flips the signal upside down.
The gain comes from transconductance, gm, which says how much drain current a given gate voltage commands. The voltage gain is approximately minus gm times the drain resistance: Av = -gm times Rd. For example, with gm = 2 millisiemens and Rd = 5 kilohms, the gain is about -(0.002 times 5000) = -10, so a 0.1 V input swing becomes a 1 V inverted output swing. To work, the MOSFET must be biased in its saturation region so the drain current is a clean function of gate voltage.
The common-source stage offers a near-infinite input resistance (the insulated gate draws no current), which is its great advantage over the common-emitter BJT, so it barely loads the source feeding it. The trade-offs: its output resistance is roughly the drain resistor, so it does not drive heavy loads well, and like any high-gain inverting stage it is sensitive to bias drift and needs source degeneration or feedback for a stable operating point. It is the go-to gain stage when input loading must be minimal.
A common-source stage with gm = 2 mS and a 5 kilohm drain resistor gives a gain of about -10: feed in 50 mV peak and you get 0.5 V peak out, inverted.
Gain is minus gm times the drain resistor; the output inverts.
A MOSFET's gm is lower than a BJT's at the same current, so a common-source stage usually gives less gain than a common-emitter one. You trade some gain for the near-infinite input resistance.