In a PMOS transisor, the drain to source current is a function of all the terminal voltages
,
and
The drain current in a PMOS transistor is given by
(1)
Small signal current is given by
(2)
transconductance
backgate transconductance
output conductance
The small signal model in Figure 1(b) is much helpful while circuit analysis.
From Figure 1(b), we can say small signal model of PMOS appears almost same as NMOS, except the direction of drain current.