# Diode ac and dc resistance?

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 dont know how to find the DC resistance of the diode can anyone help me out

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Gold Member

Are you sure that differentiation is right? doesn't look correct.

i think its correct .......what i have done is after differentiation i have subb i(forward) to the term I(saturation)*exp(ev/nKT).....and im sure of that because i have done it before ..........but i dont know how to calculate DC resistance ...DC(resistance)=V/I .......am i correct ...........but i dont how to get a value for that ................

Gold Member

Yea sorry its correct. I thought doing that gave DC resistance not AC. Also do you know the foward voltage.

No i dont ,,,,,,,,,but after referring to some of my books i got some idea ......though i dont have the forward voltage i might be able to calculate it using forward and reverse currents by subbing them in the above equation then can get forward voltage ............

so i think DC(resistance)=V(forward)/I(forward)............

can someone check this answer for me ....................here i forward is 20mA i saturated is 2uA and temp is 300k.............i got the answer as R(dc)=11.9 ohms ........am i correct ????????please someone check and tell me because in the MCQ question there is no such answer ..............:(

Gold Member

I'll ask my teacher. That sounds a bit high though.

can someone check this answer for me ....................here i forward is 20mA i saturated is 2uA and temp is 300k.............i got the answer as R(dc)=11.9 ohms ........am i correct ????????please someone check and tell me because in the MCQ question there is no such answer ..............:(