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## Re: Dynamic Resistance

 Subject: Re: Dynamic Resistance "Andrew Holme" 20 Mar 2006 03:52:25 -0800 sci.electronics.basics
 ```Seeker wrote: > 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? 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. ```
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