# AC resistance of a diode

Discussion in 'General Electronics Discussion' started by synaptic, Jul 27, 2010.

1. ### synaptic

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Mar 15, 2010
Hello, just have a quick question. What is AC resistance of a diode? I understand a diode has a non-linear IV response, but if you apply an AC waveform to a diode, the diode will rectify it, so wondering what is meant by AC resistance in this case.

2. ### Resqueline

2,848
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Jul 31, 2009
Here is a thread discussing what I believe you are referring to (dynamic impedance). A very small AC wave imposed onto a larger DC level won't be rectified.

3. ### NickS

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Apr 6, 2010
Its a trick question right? You almost had me but then I remembered resistance does not change with frequency so it is the same as the DC resistance.

But you were probably meaning to ask about the impedance which will change because reactances do change with frequency.

4. ### Resqueline

2,848
2
Jul 31, 2009
It would seem to me it could be a student needing an answer to a task/test. By "change with frequency " you just mean DC-AC?
Well, I think the question is valid, but the answer is in where the applied waveform instantaneously "hits" on the I/V curve of the diode.
For AC the instantaneous resistance is continously changing, but I guess one could integrate it and come up with a mean reactance (or whatever it should be called).
But when one is interested in the diode impedance (or AC resistance for the uninformed) it's usually about the steepness of the steep part of the diode characteristics.
I guess Dave would know about using diodes as relays/switches in these applications, their RF impedance being controlled by a DC current being passed through them.
I believe this is what (balanced) mixers/modulators make use of. And there's something about the name PIN-diodes that rings a bell in this context.

5. ### Laplace

1,252
184
Apr 4, 2010
From my textbook on analysis of the p-n junction: The static resistance of a diode is defined as the ratio V/I of the operating point voltage and current. The static resistance varies widely with the V and I and is not a useful parameter. For small-signal operation the dynamic, or incremental, resistance r is an important parameter, and is defined as the reciprocal of the slope of the volt-ampere characteristic, r = dV/dI. Although r varies inversely with current, in a small-signal model, it is reasonable to use the parameter r as a constant.

Note that the dynamic resistance is just simple resistance and not a reacitve impedance. I believe that diode AC resistance is the same as the dynamic resistance.