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I have been googling about it and I see lots of different ways such as only flipping the signs (< becomes >) and switching variables (VGS becomes VSG). Some people even say just put absolute value without changing anything and putting absolute for threshold voltage. Which one is most accurate and easy to remember?

I know that the current ID goes from source to drain so we have to flip the AC model also.

I just need an easy hint of altering the NMOS equations for PMOS to keep in mind for my exam.

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Absolute value is the easiest way to remember, for me.

The NMOS needs positive voltage between Gate and Source to conduct positive current from drain to source (with drain at higher voltage than source). The PMOS needs negative voltage between the gate and Source to conduct negative current from drain to source (with drain at a lower voltage than source).

So, they are just duals of one another.

And the polarities of the current and voltage become very obvious, especially if you draw the FET in place with voltages and currents shown.

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    \$\begingroup\$ .... and AC small signal models are identical 😉 \$\endgroup\$ – carloc Aug 17 '16 at 15:29

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