# What is the formula for dynamic resistance of a diode?

#### acamarujin

Joined Oct 20, 2015
1
I found 2 equations for dynamic resistance of a diode and I want to know the difference between them
1.
Rd=KT/q(I+Is)
2.
rd=nVT/IdQ

I just started learning about analog electronics in college and I couldn't find anything that it's complementary to my courses
For example I couldn't find anything about Small-signal equiv circuit
and if I could find some solved problems it will be wondeful

#### WBahn

Joined Mar 31, 2012
30,286
Use the notion that, around a particular large-signal operating point, Vo and Io, the instantaneous voltage and current, v(t) and i(t), can be written as the sum of the large-signal values and the small-signal values:

v(t) = Vo + Δv(t)
i(t) = Io + Δi(t)

For sufficiently small deviations from the large-signal operating point, the relationship between Δv(t) and Δi(t) can be approximated as being linear such that

Δv(t) = rd·Δi(t)

To find rd, simply start with the basic constitutive equation for a p-n junction diode and apply the above notion.