Hi. I've been trying to understand the solution to a practice problem in my book (it only provides the answer), but I cant seem get it. The problem gives the circuit in the attachment and asks the following: Find a value of R such that V0 is equal to 2.4 V. Assume that the diodes available have 0.7 V drop at 1 mA.

The solution is R = 139 ohms. I keep getting about 160. If someone could tell me how I should go about solving this it would be great! Thanks.

 

You forgot to attach the diagram.

 

Here it is.

 

Te get the right answer you need to use Shockley diode equation
\( Id = Is * e^{\frac{Vd}{26mV}}\)
First you need to find Is current, because you know Id = 1mA; Vd= 0.7V so
Is = 2.03fA
Now you want Vo = 2.4V but you have three diodes so Vd = 2.4V/3 = 0.8V
Id = 2.03fA * e^(0.8V/26mV) = 46.816mA
So we know Id current which causes 0.8V voltage drop across the diode.
And now we can find resistor R.
R = (10V - 2.4V) / 46.816mA = 162Ω

So I get the same answer as you.

 

I support Jony130's method of solution.