Mos amplifier numerical
Using KCL in the output node, you find that

$$I_X = i_{DS1} - i_{DS2}$$

(I'm using positive quantities for the PMOS here).

You can see that the current is negative if \$i_{DS1} &lt; i_{DS2}\$. Otherwise, the current is positive. When is this the case?

For small \$V_X\$, M1 will not conduct a lot of current because of the small \$v_{DS}\$ applied. If \$V_X &lt; v_{GS1} - V_{TH1}\$ then M1 is in the triode region.

For large \$V_X\$ we have the same situation for M2. It is in the triode region.

If \$V_{DD}\$ is large enough for one or both of the transistors to go into saturation, then the situation can change a little bit.


M1 in saturation, M2 in triode

M1 will behave like a current source, while M2 can be better approximated by a resistor. So the slope will be somewhat linear, but M1 will add a relatively constant current to \$I_X\$.
M1 in triode, M1 in saturation

The situation is identical, but the slope will be somewhat linear while M2 will subtract a constant current.
M1 and M2 are both in saturation

Both transistors act more or less like a current source, so the net current \$I_X\$ will be pretty much constant. However, because both transistor currents are slightly dependent on \$v_{DS}\$ (ie. output impedance), \$I_X\$ will not be perfectly constant.