I don't ask this. When you wnat to plot Vth vs Vgs ( the Vs in not grounded!!) what procedure (steps) do you follow? I want to someone to explain me such like that:
1 Step: Go there and put "this"
2 Step: Go to DC Analysis and put this variable
3 Step: .....
4 Step: .....
what do you want? gate voltage is fixed and sweeping source voltage? what is bulk connected to, ground or source? make your test fixture clear otherwise you will not get an answer.
The circuit topology is a basic differential pair implemented with NMOS transistors (MN1,MN2). The bulk of MN1, MN2 is grounded. I want to plot Vth vs Vin_CM (CM=Common Mode).
If you want a rough VTH, set up a single transistor in
a similar headroom and body-voltage configuration with
a small current source pulling on the MN1.S terminal.
A 1uA/sq value ought to be close to VT.
Then just sweep the gate and take Vgs as "close enough"
to VT at any point.