Editing Common drain (section)

{{Short description|Electronic amplifier circuit type}}
{{More footnotes|date=April 2009}}
In [[electronics]], a '''common-drain''' [[electronic amplifier|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 amplifier|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. Because of its low dependence on the load resistor on the voltage gain, it can be used to drive low resistance loads, such as a speaker.  The analogous [[bipolar junction transistor]] circuit is the [[common collector|common-collector amplifier]]. This circuit is also commonly called a "stabilizer".

In addition, this circuit is used to transform [[Electrical impedance|impedances]]. For example, the [[Thevenin's theorem|Thévenin resistance]] of a combination of a voltage follower driven by a voltage source with high Thévenin resistance is reduced to only the output resistance of the voltage follower (a small resistance). That resistance reduction makes the combination a more ideal voltage source. Conversely, a voltage follower inserted between a driving stage and a high load (i.e. a low resistance) presents an infinite resistance (low load) to the driving stage—an advantage in coupling a voltage signal to a large load.