# Diode circuit design problem

Discussion in 'Homework Help' started by Alice, Mar 23, 2009.

1. ### Alice Thread Starter New Member

Mar 29, 2008
5
0
Hi,

I need help on the following problem:

I can't attach an image of the circuit, but it's pretty straight-forward. There's a 15 V dc source in series with a resistor R of unknown resistance, and in series with a string of four forward biased diodes going to ground. The circuit output Vo is across the string of 4 diodes.

Design the circuit so that Vo = 3V when IL = 0 (the load current is zero) and Vo changes by 40mV per 1mA of load current.

I need to find the value of the resistance R and the junction area of each diode relative to a diode with a 0.7 V drop at 1mA. n = 1.

I've tried several different methods and nothing is producing the correct answer. (the solution is provided, R = 4.8KΩ and junction area = 0.34)

I'm not sure if using the small-signal diode model is justified in this case since it seems that the change in voltage per 1 mA per diode is too great. However when I try using the exponential model, using the equation v2-v1 = 2.3nVTln(I2/I1) and a KVL equation to solve for the resistance R, I don't get the right answer either.

Any help would be greatly appreciated!... and I really do need this by tomorrow:/

Thank you,

Alice.

Last edited: Mar 23, 2009
2. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
No they aren't.

3. ### Alice Thread Starter New Member

Mar 29, 2008
5
0
yeah... hence the recent editing of the post sorry 'bout that