> I stumbled upon the notion of dynamic resistance of the p-n junctions.
> I'm pretty good at calculus so I understand what dynamic resistance is.
> What I don't understand though is why we take it as the resistance of,
> say, a diode for small ac voltages instead of the static resistance.
> Isn't the current through the diode the voltage over its static
> resistance? Why do we use the dynamic resistance which is essentially the
> rate of change of the current in regards to the voltage? Why do we care
> about the rate of change and don't care about the value of the resistance
v(t) = V + dv
i(t) = I + di
Static resistance R = (V + dv) / (I + di) which is approximately V / I
Dynamic resistance r = dv/di which could be very different from R
When the current through the PN junction changes by a small amount di,
the resultant change in voltage is r*di not R*di.