Hi, I'm having trouble plotting the tangent line on a curve (or the derivative evaluated at a point) in spice. I know how to plot the derivative of the entire curve using the d() function, but I don't know how to evaluate it at a point. Any help is much appreciated!

 

You use the definition of the derivative at a point

f'(x) = (f(x+h)-f(x))/h

As the constant h approaches zero the approximation to the derivative becomes better and better.

 

OK. How do I enter that in the "Add trace" window though? For example, the name of my function f is IB(MPS3704). I can't do something like IB(MPS3704)(x+0.6). I thought the "@" operator would help, but an expression like IB(MPS3704)@50uA was invalid too. Essentially, if I could find out how to use the value of a function at a point, that would solve a lot of problems. Anybody know how to do this?