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.
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.
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.
More Circuits & Circuit Design:
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Transistor Darlington
Transistor circuits
FET circuits
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