Common drain

In electronics, a common-drain amplifier, also known as a source follower, is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage buffer.

In this circuit (NMOS) the gate terminal of the transistor serves as the signal input, the source is the output, and the drain is common to both (input and output), hence its name.

The analogous bipolar junction transistor circuit is the common-collector amplifier.

That resistance reduction makes the combination a more ideal voltage source.

[1] The variable gm that is not listed in the schematic is the transconductance of the device (usually given in units of siemens).