Calculating AC and DC Resistance of a Silicon Diode in 300K

[rclakmal]

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

 

[madmike159]

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

 

[rclakmal]

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 I am sure of that because i have done it before ...but i don't know how to calculate DC resistance ...DC(resistance)=V/I ...am i correct ...but i don't how to get a value for that ...

 

[madmike159]

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

 

[rclakmal]

No i don't ,,,,,,,,,but after referring to some of my books i got some idea ...though i don't 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)...

 

[rclakmal]

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 .....:(

 

[madmike159]

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

 

[overt26]

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 .....:(

I calculated 11.97 ohms which agrees well enough with your answer.

 

[rclakmal]

Thnaks dude !it was a great help!