FET Common Source Amplifier Circuit

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.

Common source FET amplifier characteristics summary

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

** 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.

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.

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