CMOS Analog IC Review Part I

3.4. Source Follower

Source follower stage can act as a buffer with approximately unity gain. Small-signal analysis yields:

A_v = g_mr_OR_S/(r_O + R_S + g_mr_OR_S), or

Av = g_mR_S/[1 + (g_m + g_mb)R_S] if we neglect r_O but consider body effect.

I_D heavily depends on the input DC level, which incurs nonlinearity. This can be alleviated by replacing RS with a constant current source.

The output resistance is

R_out = 1/(g_m + g_mb).

In the following configuration,

Source followers suffer from nonlinearity due to body effect. Using PMOS source follower with separate n-wells and tying bulk to the source can eliminate body effect. 

Source followers also shift the DC level of the signal by V_GS, thereby consuming voltage headroom and limiting the voltage swings.