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I am stuck at a point and needs help.

The thing is I am doing MOSFET inverters using Pspice.

I am getting its VTC (voltage characteristic Curve) But I don't know how to get derivative of this curve in order to obtain some parameters required for Noise margin.

Here is the file:

.model NMOD NMOS (LEVEL=1, VTO=0.6 GAMMA=0 LAMBDA=0 KP=60U)

.model PMOD PMOS (LEVEL=1, VTO=-0.8 GAMMA=0 LAMBDA=0 KP=20U)

***********

* CIRCUIT *

***********

VDD 3 0 DC 3

VGS 1 0 DC 3

M1 2 1 0 0 NMOD L=0.8U W=2U

M2 2 1 3 3 PMOD L=0.8U W=10U

.OP

.DC LIN VGS 0 3 0.1

.PLOT DC V(2)

.PROBE

.END

I have 9.1 Student ,Demo version. I referred to user manual... and it says I used use probe function 'd()' to get derivative but that's not working. It says invalid expressions.

I am taking 'd()' at 'V(2)'.

I would really appreciate if some one can help me out.

Thanks,