FET Common Drain / Source Follower

The common drain or source follower circuit is able to provide a very high input impedance and low output impedance and is used to act as a buffer amplifier.

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

The like the transistor emitter follower, the FET 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.

FET common drain configuration showing how the drain is common to both input and output circuits — Common drain (source follower) FET circuit configuration

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 resistance	Very High
Output resistance	Low
Input / output phase relationship	0°

Source follower / buffer circuit

The typical implementation of the common drain or source follower / buffer circuit is very easy to realise in a practical fashion.

The circuit show below gives a typical example of a FET source follower / buffer circuit. The capacitors C1 and C2 are used to couple the AC signal between stages and block the DC elements. The resistor R1 provides the gate bias, holding he gate at ground potential. The source circuit shows the resistor R2 to ground - its value is determined by the channel current that is required.

FET common source circuit showing bias and coupling arrangements — FET common source circuit

The source follower circuit presents a very high impedance to the preceding stage and it is for this reason that the source follower is an ideal format for use as a buffer.