FET Common Drain / Source Follower

- details of the common drain or source follower circuit that can be designed using a field effect transistor, FET.

Common drain or source follower is an excellent FET circuit.

The like the transistor emitter follower, the source follower configuration itself provides a high level of buffering and a high input impedance. The actual input resistance of the FET itself is very high as it is a field effect device. This means that the source follower circuit is able to provide excellent performance as a buffer.

The voltage gain is unity, although current gain is high. The input and output signals are in phase.


Diagram of the common drain / source follower FET buffer


Source follower amplifier characteristics summary

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


Common drain, source follower FET amplifier characteristics
Parameter Amplifier Characteristics
Voltage gain Zero
Current gain High
Power gain Medium
Input / output phase relationship
Input resistance Very High
Output resistance Low

By Ian Poole


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