JFET as an amplifier- The weak signal is applied between gate and source, and amplified output is obtained across drain and source terminal.
For proper operation of JFET gate must be negative with respect to source. Input circuit should always be reversed biased with help of a battery Vgg or by a circuit is known as biasing circuit.
A small change in the reverse bias on the gate produces a large change in drain current. This make JFET capable of raising the strength of a weak signal.
During positive half cycle of signal the reverse bias on the gate decreases. This increases the channel width and hence the drain current.
During negative half cycle of the signal reverse voltage on the gate increases. Hence drain current decreases. The result is that the small change in voltage at the gate produces large change in drain current. This large variation in drain current produces large output across the load. In this way JFET acts as an amplifier.
Also Read- Working of JFET