# Dynamic Resistance

Discussion in 'Electronic Basics' started by Seeker, Mar 20, 2006.

1. ### SeekerGuest

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
itself?

2. ### Phil AllisonGuest

"Seeker"

** Both resistances are variables with current - unlike a normal conductor
where the resistance is a fixed value (for constant temp).

The ratio of voltage to current is one variable for a given diode (again,
with fixed temp).

The ratio of a small change ( or increment ) in voltage to the resulting
change is current is another variable for the same diode.

The latter will usually have a much lower value.

........ Phil

3. ### Andrew HolmeGuest

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.

4. ### John PopelishGuest

If the signal of interest is AC, and is small, compared to the DC bias
voltage of the diode, then the incremental resistance deals with what
the diode does with the small AC signal (how its resistance absorbs
power from that signal).