Common Drain Stage or Source Follower
When the MOSFET is operating in the Common Source stage, for proper amplification it needs a resistor of high value at the output.
In other words the load impedance must be as large as possible.
Now if the common source stage has to drive a low impedance load, then we need a buffer in between so that there is no loss of gain.
Under these conditions the Common Drain stage can be used as a buffer in between. A common drain stage is also popularly called as Source Follower.
Figure 8 shows the circuit diagram of a Source Follower.
Figure 8: Common Drain Stage / Source Follower
We can use the Source follower as a Buffer, figure 9 illustrates the same
Figure 9: Illustration of Source Follower as Buffer