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**.