How is the junction area of each diode calculated in this circuit?

[shaiqbashir]

Hi Guys!

My Question is this:

Dsign the circuit of the fig: so that Vo= 3V when IL = 0 A AND Vo changes by 40mV per 1mA of load current. the value of R = 4.8Kilo ohms. find the juction area of each diode (assume all four diodes are identical) relative to a diode with 0.7 V drop at 1mA current. Assume n=1.

The ans of this question is 0.34 but how it has been calculated.

Plz help
Thanks in advance:

 

[nalini pareek]

since we know that I(s) , i.e. the saturation current is proportonal to the junction area , calculate saturation current for the two cases as follows:
first for the four diodes ,given that they are identical, the voltage drop across each will be 3/4 = .75v, I = 2.5 mA. using eq I = I(s)exp(v/nVt) we get saturation curret for the given diode as 2.339 x 10^(-16).
Similarly for the given standard diode that is one with the voltage drop of .7v and I = 1mA
we get saturation current as 6.19 x 10^(-16).

since we have to find the relative area, simply dividing the above two saturation current would give the answer that is .34