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I have a simple NMOS FET where I would like to plot the differential output resistance at the operating point of Vgs = 1.7V (which is the Vgs voltage for Id=9.4mA)

I managed to plot the output resistance with the help of the behavioural source B1 as follows:

enter image description here

I can read from the difference of cursors 1 and 2 in the plot that the differential resistance should be about 2386 Ohms at Vgs ~= 1.7V.

Main question: How can I get a plot of this differential resistance? Is there for example a way to plot the derivative of a trace in LTSpice?

Bonus question: How could I plot the small signal gain of this amplifier?

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  • \$\begingroup\$ How could be "out" a variable ? \$\endgroup\$
    – Antonio51
    Commented Jan 11, 2024 at 13:12
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    \$\begingroup\$ You don’t need to use the behavioral source. You can do waveform arithmetic directly on the data. Use the function d() to take the derivative. \$\endgroup\$
    – Ste Kulov
    Commented Jan 11, 2024 at 13:49

1 Answer 1

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As proposed by @Ste Kulov, waveform arithmetic is exactly what I was looking for. I took the derivative function d(V(out)/Id(M1)).

enter image description here

Concerning the small signal gain, this can be done by right-click on V1, specify a (small) AC amplitude like 0.01V and change the simulation command to .ac and desired frequency range.

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  • \$\begingroup\$ You're missing an end parenthesis ) in your answer. Also, are you able to handle the gain now or do you still need help there? \$\endgroup\$
    – Ste Kulov
    Commented Jan 12, 2024 at 6:00
  • \$\begingroup\$ Thanks, I fixed the ). How to get the small signal gain is still unclear. Large signal gain would be V(out)/V1? \$\endgroup\$
    – Stefan Wyss
    Commented Jan 12, 2024 at 7:42
  • \$\begingroup\$ I think what you might be looking for is this. Right-click V1 and hit "Advanced". Type 1 for "AC Amplitude" and press OK. Then change your simulation command to .ac. Select a frequency range, or if you just want one frequency then select "List" and only type one in. If you want to sweep V1's DC voltage while doing this, then look into the .step param command. \$\endgroup\$
    – Ste Kulov
    Commented Jan 12, 2024 at 7:56

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