Mode of Operation of JFET

Modes of operation of FET’s

Like the bipolar junction transistor, the field effect transistor works in three modes of operation and can be connected within a circuit in one of the following configurations.

1.       Common Source (CS) Configuration

In the Common Source configuration same as a common emitter, the input is applied to the Gate and its output is taken from the Drain as shown in the circuit above. This is the commonly used mode of operation of the FET because of high input impedance and good voltage amplification and as such Common Source amplifiers are widely used.

This mode of operation is generally used audio frequency amplifiers and in high input impedance pre-amps and stages.

2.       Common Gate (CG) Configuration

In the Common Gate configuration same as a common base, the input is applied to the Source and its output is taken from the Drain with the Gate connected directly to ground (0v) as shown in above figure. In this configuration as the common gate has a low input impedance, but a high output impedance.

This type of FET configuration commonly used in high-frequency circuits or in impedance matching circuits was a low input impedance needs to be matched to a high output impedance.

3. Common Drain (CD) Configuration

In the Common Drain configuration same as a common collector, the input is applied to the Gate and its output is taken from the Source. The common drain or it is also called “source follower” configuration has a high input impedance and a low output impedance and almost unity voltage gain so is therefore used in buffer amplifiers. The voltage gain of the source follower configuration is less than unity, this type of configuration is term as “Common Drain” because there is no signal available at the drain connection, the voltage present, +V_DD just provides a bias.