# Need help finding a derivative

1. Apr 10, 2010

### zckumling

Hi, this is my first post on this forum ..

My education is ship-electrician, I'm not starting calculus until next semester when I start on my engineering degree .. I've started reading stuff on my own and I'm doing a circuit simulation ..

Here's my function: (KCL)
$${\frac {{\it vaa}-{\it vout}}{{\it ra}}}-{\it lookupIa} \left( {\it vgk} \right) -{\frac {{\it vout}}{{\it rload}}}=0$$

I need help to find the derivative of this function to use it with a newton method iteration..

lookupIa(vgk), is my lookuptable in C++ and I have no clue how to solve this function when it depends on a second function.. ;)

I'm a complete idiot on this subject at the moment, any help on this and I'm forever grateful!

2. Apr 13, 2010

### Redbelly98

Staff Emeritus
Welcome to Physics Forums

I looked at your question a couple of days ago, but am having a little trouble understanding it. Some things that are not clear to me are:
What is "this function" that you want to find the derivative of? lookupIa(vgk), or Vout, or the entire left-hand-side of the equation? Or something else?

What are you taking the derivative with respect to, is it time or something else?

What is vgk?​
That being said ... I don't know if this helps any, but a pretty good estimate of the derivative of f(x), when f is given in a table, would be
f'(xi) ≈ (f(xi+1) - f(xi-1)) / (xi+1 - xi-1)

By the way, thanks for providing a little background about yourself. I find it's easier to help somebody if I know a little about where they're coming from.