Q. Draw the circuit of a common drain FET amplifier and explain.

FET amplifier circuit

The weak signal is applied between gate and source and amplified output is obtained in the drain-source circuit. For the proper operation of FET, the gate must be negative w.r.t source i.e. input circuit should always be reverse biased. This is achieved either by inserting a battery VGG in the gate circuit or by a circuit known as biasing circuit.

A small change in the reverse bias on the gate produces a large change in the drain current. This fact makes FET capable of raising the strength of a weak signal. During the positive half of signal, the reverse bias on the gate decreases. This increases the channel width and hence the drain current. During the negative half cycle of the signal, the reverse voltage on the gate increases. Consequently, the drain current decreases. The result is the small change in voltage at the gate produces a large change in the drain current. These large variations in drain current produce large output across the load RL. In this way, FET acts as an amplifier.

There are three basic FET amplifier configurations. They are:

· Common source (CS)

· Common drain (CD)

· Common gate (CG)