FET Circuit Configurations

FET circuit configurations are the common source, common gate & common drain formats which each have their own characteristics.

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

FET circuit configurations are the common source, common gate, and common drain formats. Each have their own characteristics of voltage and current gain as well as input and output impedance.

The choice of the FET circuit configuration or topology is one of the key design parameters on which the overall circuit design is based.

FET configuration basics

The terminology used for denoting the three basic FET configurations indicates the FET electrode that is common to both input and output circuits. This gives rise to the three terms: common gate, common drain and common source.

The three different FET circuit configurations are:

  • Common source:   This FET configuration is probably the most widely used. 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 drain:   This FET configuration is also known as the source follower. The reason for this is that the source voltage follows that of the gate. Offering a high input impedance and a low output impedance it is widely used as a buffer. The voltage gain is unity, although current gain is high. The input and output signals are in phase.
    FET common drain configuration showing how the drain is common to both input and output circuits
    Common drain (source follower) FET circuit configuration
  • Common gate:   This transistor configuration provides a low input impedance while offering a high output impedance. Although the voltage is high, the current gain is low and the overall power gain is also low when compared to the other FET circuit configurations available. The other salient feature of this configuration is that the input and output are in phase.
    FET common gate configuration showing how the gate is common to both input and output circuits
    Common gate FET circuit configuration

FET circuit configuration summary table

The table below gives a summary of the major properties of the different FET circuit configurations.

FET Configuration Summary Table
FET Configuration Common Gate Common Drain
(Source follower)
Common Source
Voltage gain High Low Medium
Current gain Low High Medium
Power gain Low Medium High
Input resistance Low High Medium
Output resistance High Low Medium
Input / output phase relationship 180°

As can be seen the different configurations or topologies have different characteristics. The common source is the most widely used FET circuit configuration and it equates to the common emitter transistor amplifier. The FET common drain or source follower is used as a buffer amplifier and it equates to the transistor common emitter amplifier.

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