FET Common Gate Amplifier Circuit

FET common gate amplifier is used to provide low input impedance or high isolation between input & output to stop oscillation, etc.

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

The FET common gate amplifier circuit is the least widely used, but it does possess some characteristics that can be put to good use in some applications.

As such the FET common gate circuit is seen used in a limited number of applications, although the common source and common drain configurations are sued far more widely.

FET common gate features

Common gate FET configuration provides a low input impedance while offering a high output impedance. As the gate is grounded, this acts as a barrier between input and output providing high levels of isolation, preventing feedback, especially at very high frequencies.

Although the voltage gain 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 common gate applications

The FET common gate circuit is not as widely used as other FET configurations, as it often provides very few advantages over other configurations. There are some areas where it proves to be very useful.

RF amplifiers: The common gate or grounded gate circuit configuration is used for VHF and UHF RF amplifiers where the low input impedance allows accurate matching to the feeder impedance which is typically 50Ω or 75Ω. The configuration also improves stability which is a key issue. The gate, being grounded provides isolation between the input and output, thereby considerably reducing the possibility of feedback.
Microphone amplifiers: The common gate circuit configuration does find uses with amplifiers that require low input impedance levels. One application is for moving-coil microphones preamplifiers - these microphones have very low impedance levels.

Common gate amplifier characteristics summary

The table below gives a summary of the major characteristics of the common gate amplifier circuit.

Common gate transistor amplifier characteristics
Parameter	Characteristics
Voltage gain	High
Current gain	Low
Power gain	Low
Input / output phase relationship	0°
Input resistance	Low
Output resistance	High