Editing Common source (section)

{{Short description|Electronic amplifier circuit type}}
{{More footnotes|date=January 2018}}

[[Image:N-channel JFET common source.svg|frame|Figure 1: Basic N-channel JFET common-source circuit (neglecting [[biasing]] details).]]
[[Image:N-channel JFET common source degeneration.svg|frame|Figure 2: Basic N-channel JFET common-source circuit with {{Clarify|date=May 2025|text=source degeneration}}.]]
In [[electronics]], a '''common-source''' [[electronic amplifier|amplifier]] is one of three basic single-stage [[field-effect transistor]] (FET) amplifier topologies, typically used as a [[Electronic amplifier#Ideal|voltage or transconductance]] [[amplifier]]. The easiest way to tell if a FET is common source, [[common drain]], or [[common gate]] is to examine where the signal enters and leaves. The remaining terminal is what is known as "common". In this example, the signal enters the gate, and exits the drain. The only terminal remaining is the source. This is a common-source FET circuit. The analogous [[bipolar junction transistor]] circuit may be viewed as a transconductance amplifier or as a voltage amplifier. (See [[Electronic amplifier#Ideal|classification of amplifiers]]). As a transconductance amplifier, the input voltage is seen as modulating the current going to the load. As a voltage amplifier, input voltage modulates the current flowing through the FET, changing the voltage across the output resistance according to [[Ohm's law]].  However, the FET device's output resistance typically is not high enough for a reasonable transconductance amplifier ([[Electronic amplifier#Ideal|ideally infinite]]), nor low enough for a decent voltage amplifier ([[Electronic amplifier#Ideal|ideally zero]]). As seen below in the formula, the voltage gain depends on the load resistance, so it cannot be applied to drive low-resistance devices, such as a speaker (having a resistance of 8 ohms).  Another major drawback is the amplifier's limited high-frequency response. Therefore, in practice the output often is routed through either a voltage follower ([[common-drain]] or CD stage), or a current follower ([[common-gate]] or CG stage), to obtain more favorable output and frequency characteristics. The CS–CG combination is called a [[cascode]] amplifier.