FET Common Source Amplifier Circuit

The common source FET amplifier circuit is one of the most commonly used providing current and voltage gain along with a satisfactory input and output impedance.

FET, Field Effect Transistor Circuit Design Includes:
FET circuit design basics Circuit configurations Common source Common drain / source follower Common gate

Common source FET configuration is probably the most widely used of all the FET circuit configurations for many applications, providing a high level of all round performance.

The common source circuit provides a medium input and output impedance levels. Both current and voltage gain can be described as medium, but the output is the inverse of the input, i.e. 180° phase change. This provides a good overall performance and as such it is often thought of as the most widely used configuration.

FET common source configuration showing how the source is common to both input and output circuits — Common source FET circuit configuration

Common source FET amplifier characteristics summary

The table below gives a summary of the major characteristics of the common source amplifier.

Common source amplifier characteristics
Parameter	Amplifier Characteristics
Voltage gain	Medium
Current gain	Medium
Power gain	High
Input / output phase relationship	180°
Input resistance	Medium**
Output resistance	Medium

** Note: the input resistance for a FET itself is very high in view of the fact that it takes virtually no current.

Typical common source amplifier circuit

The circuit below shows a typical common source amplifier with the bias as well as the coupling and bypass capacitors included.

Basic FET common source amplifier showing bias arrangements and the bypass and coupling capacitors — Basic FET common source amplifier

The input signal enters via C! - this capacitor ensures that the gate is not affected by any DC voltage coming from the previous stages. The resistor R1 holds the gate at ground potential. T value could typically be around 1 MΩ. The resistor R2 develops a voltage across it holding the source above the ground potential. C2 acts as a bypass capacitor to provide additional gain at AC.

The resistor R3 develops the output voltage across it, and C3 couples the AC to the next stage whilst blocking the DC.