CMOS Analog IC Review Part I

3.5. Common-Gate Stage

Small-signal analysis yields

Av = [RD + (gm + gmb)RDrO]/(RD + rO), or

Av = (gm + gmb)RD if we neglect rO.

Take into account the source resistance RS

The output resistance for common-gate stage is 

Rout = [rO + RS + (gm + gmb)rORS]||RD.