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**Diode ac and dc resistance?**

here is my question ...saturation current of a silicon diode is 2uA.and forward current is 20mA...i want to find out the AC and DC resistances of the diode in 300k...

so i know that ,

I(forward)=I(saturate)[exp(eV/nKT)-1]

..so by differentiating both sides w.r.t to V i got that dI/dV=I(forward)*e/nkt...here KT/e=26mv so by subbing than (and also n=1 when conducting through diode)i got dV/dI=1.3ohms ...

so that is Ac resistance of the diode ...

but i don't know how to find the DC resistance of the diode can anyone help me out