Common Drain JFET Amplifier

Fig. 25 shows the circuit diagram of common drain jfet amplifier or source follower. The output is taken between the source and the ground.

Expression for common drain jfet amplifier

The equivalent circuit of fig. 25 is shown in fig. 26.

Since r_d and R_S are in parallel, so their equivalent resistance is given by

Output voltage V_o for common drain jfet amplifier is given by

Using eqn. (1), we get

The input signal applied between gate and drain is represented by the input voltage V_i which is given by

Now, voltage gain (A_v) is given by

Since r_d >> R_S, so eqn. (3) can be written as

Eqn. (4) shows that voltage gain is slightly less than unity. Since output voltage at the source terminal follows variation in the signal voltage applied to the gate, so common drain circuit is also known as source follower.