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.