Common Drain Amplifier

In electronics, a common-drain amplifier, as well termed as a source follower, is one type of three basic single-stage field effect transistor (FET) amplifier topologies, commonly employed as avoltage buffer. In this type of circuit the gate terminal of the transistor works as the input, the source is the output, and the drain is general to both (input and output), therefore its name. The analogous bipolar junction transistor circuit is common-collector amplifier.

Additionally, this circuit is employed to convert impedances. For instance, the Thévenin resistance of a combination of a voltage follower driven via a voltage source along with high Thévenin resistance is decreased to only the output resistance of the voltage follower, a small resistance. The resistance reduction created combination a much more perfect voltage source. On the other hand, a voltage follower inserted in between a driving stage and a high load (that is a low resistance) represents an infinite resistance (low load) to the driving stage, a benefit in coupling a voltage signal to a large load.