Small signal Analysis of MOSFET
This configuration serves as the gain stage. The disadvantage is high output impedance. Capacitor CS is included such that the stage is connected to a current source for biasing
This amplifier provides gain and is useful when a specific (low) Rin is required. This is, e.g., the case when the impedance needs to be matched, as with transmission lines (e.g. to 50 Ω). Another application of the CG configuration is that it acts as a current buffer (current gain close to unity, small Rin, large Rout).
Source Follower (Common-Drain Configuration)
This configuration acts as a voltage buffer. It provides no gain, but has low output impedance. It is typically the last stage in a multi-stage amplifier.