Diode circuit design problem

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Joined Mar 29, 2008

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,

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