Encyclopedia

Common drain

Electronic amplifier circuit type

Wikipedia contributorsAdapted for Fathomly · View authors

1 min read
Source updated April 2, 2025

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. 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 amplifier. This circuit is also commonly called a "stabilizer".

In addition, this circuit is used to transform impedances. For example, the 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.

01Characteristics

At low frequencies, the source follower pictured at right has the following small-signal characteristics.

	Definition	Expression	Approximate expression	Conditions
Current gain	$A_{\text{i}}={\frac {i_{\text{out}}}{i_{\text{in}}}}$	$\infty$	$\infty$
Voltage gain	$A_{\text{v}}={\frac {v_{\text{out}}}{v_{\text{in}}}}$	${\frac {g_{m}R_{\text{S}}}{g_{m}R_{\text{S}}+1}}$	$\approx 1$	$g_{m}R_{\text{S}}\gg 1$
Input impedance	$r_{\text{in}}={\frac {v_{\text{in}}}{i_{\text{in}}}}$	$\infty$	$\infty$
Output impedance	$r_{\text{out}}={\frac {v_{\text{out}}}{i_{\text{out}}}}$	$R_{\text{S}}\parallel {\frac {1}{g_{m}}}={\frac {R_{\text{S}}}{g_{m}R_{\text{S}}+1}}$	$\approx {\frac {1}{g_{m}}}$	$g_{m}R_{S}\gg 1$

The variable g_m that is not listed in the schematic is the transconductance of the device (usually given in units of siemens).

Watch videos about Common drainExplainers and documentaries on YouTube (opens in a new tab)

Sources and credits

This article is adapted from the Wikipedia article “Common drain”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.

Fathomly is not affiliated with or endorsed by the Wikimedia Foundation. Spotted a problem? Tell us.

Common drain

01Characteristics

Sources and credits

Related topics

Negative-feedback amplifier

Buffer amplifier

Two-port network