# Homework Help: How do I find the forward resistance of a diode using diode law?

1. Oct 9, 2011

### lilmul123

1. The problem statement, all variables and given/known data

I must find the resistance of a diode inside the piecewise model using the diode law equation.

I am given variables VD = .75V, Is = 5*10^-13 A, and VT = .026 A. n = 1

2. Relevant equations

The Diode Law equation: ID = IS(e(VD/nVT) - 1)

3. The attempt at a solution

We were told that to find the resistance, we are to find the derivative of this equation to find the conductance, and then the inverse of this would be the resistance. I'm not sure what to derive with respect to or if even this explanation is correct. Can anyone help explain how to find resistance using this equation?

2. Oct 9, 2011

### Staff: Mentor

One measure of resistance is the one that describes the operating point of the device: operating voltage divided by operating current. So if you're given the voltage across the device and can calculate the resulting current, then R = V/I.

A second measure of resistance is the 'small signal resistance', which describes how a change in voltage across the device (at the operating point) affects the current: r = Δv/ΔI. This one you might find by taking the derivative of your diode law w.r.t. voltage to find di/dv, and hence dv/di.

It looks like it's the first version of resistance you're after in this case.

3. Oct 10, 2011

### Staff: Mentor

Poster needs to find the small signal resistance. (Probably what you meant to say.)

The variables are ID and VD, being the diode current and voltage, all other terms are constants.

4. Oct 10, 2011

### Staff: Mentor

You're probably right!
Thanks for he catch.

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