Common Source Amplifier

A common-source amplifier is one of three basic single-stage field-effect transistor abbreviated as FET amplifier topologies, commonly used as a voltage or trans conductance amplifier, in electronics. The very easy way to tell if a FET is common source, common drain, or common gate is to examine in which the signal enters and leaves. The remaining terminal is what is termed as "common". In this instance, the signal enters the gate, and exits the drain. The just only terminal remaining is the source. This type of FET is a common-source FET circuit. The analogous bipolar junction transistor circuit is the common-emitter amplifier.

The common-source (CS) amplifier might be viewed like a transconductance amplifier or as a voltage amplifier. Since a transconductance amplifier, the input voltage is seen like modulating the current going to the load. Like a voltage amplifier, input voltage modulates the amount of current flowing via the FET, changing the voltage across the output resistance as per to Ohm's law. Though, the FET device's output resistance typically is not sufficiently high for a reasonable transconductance amplifier (preferably infinite), nor low enough for a decent voltage amplifier (preferably zero). Other main drawback is the limited high-frequency response of amplifier. Hence, in practice the output often is routed via either a voltage follower (common-drain or CD stage), or a current follower (common-gate or CG stage), to acquire much more favorable output and frequency characteristics. The CS-CG combination is termed as a cascode amplifier.