FET Circuit Configurations

- Circuit configurations for FET circuits including common source, common gate and common drain with their characteristics.

In just the same way that transistor (and also vacuum tube or valve circuits have various circuit configurations, so do field effect transistors.

These FET circuit configurations operate in slightly different ways giving different levels of impedance and gain.

Choosing the correct FET circuit configuration is one of the first steps in the design of any FET circuit.

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 transistor configurations are:

  • 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

    Ream more about the Common gate amplifier.
  • 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 source follower / common drain circuit showing how the drain is common to both input and putput circuits
    Common drain / source follower FET circuit configuration

    Ream more about the Common drain, source follower amplifier.
  • 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 circuit showing how the source electrode is common to both input and output circuits
    Common source FET circuit configuration
    Ream more about the Common source amplifier.

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 / output phase relationship 180°
Input resistance Low High Medium
Output resistance High Low Medium

By Ian Poole

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