john23,
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FYI, in the plot you uploaded today, the lower plotted waveform is -D(Is(M1))/D(V(Vgs)).
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But the denominator is 1.0, because the X-axis is V4, and V4 directly controls V(Vgs).? So it can be eliminated.? Perhaps you did not understand how LTspice's derivative operator D() works.? It should not be written as D(y)/D(x).? It is normally written as just D(expression).? That expression alone takes the derivative of the quantity, with respect to the X-axis variable.
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Also it looks like you are confused about whether to plot the MOSFET's Source current or its Drain current.? Yes, they are almost the same magnitude with opposite polarity, so this is more a question of principle than of numerical error.
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Andy
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