The common drain FET amplifier is similar to the common collector configuration of the bipolar transistor. A general common drain JFET amplifier, self-biased, is shown in figure 5.18. This configuration, which is sometimes known as a source follower, is characterized by a voltage gain of less than unity, and features a large current gain as a result of having a very large input impedance and a small output impedance.
Figure 5.18: JFET common drain amplifier.
Using AC circuit analysis
The voltage gain between the gate and source is
If 
, 
and we have a voltage follower.