Help with Diode Circuit with Current/Voltage through Diode

ischmob

1. The problem statement, all variables and given/known data

For the circuit shown in Fig. 4.26 both diodes are identical. Find the value of R for which V=80mV

2. Relevant equations

I'm not quite sure what equation to use here. If you look at the wiki page on diodes http://en.wikipedia.org/wiki/Diode
it might be Shockleys current equation

3. The attempt at a solution

My thought on how to approach this problem is find the current that comes out of the diode and then use that current in a voltage divider using 80mV to find a value of R. Though I have no clue what is coming out of the diode. Though I know 5mA is going to each diode.

asp_physics

Hi,
The figure is not visible. Try uploading again.

Jmf

Can you upload Fig. 4.26?

Without seeing the circuit I can't say for certain, but you may be able to use the diode equation if you know the leakage current, temperature etc. Or you may be able to solve graphically with a 'load line' if you have a plot of the diode characteristic.

ischmob

thanks for your guys prompt responses. heres another attempt at the figure upload

http://img218.imageshack.us/i/fig426.png/

Attached Thumbnails

 

asp_physics

Just a wild guess. Might be violating the laws of electricity with this answer. :) Is it 8 ohm?

I assumed the diodes to be ideal.
No current flows through the diode D2 as there is no voltage drop across it.
10mA flows through D1.
So R= 80mV/10mA = 8 ohm

ischmob

so your saying that no current will flow through D2. And as far as your answer. I think its a little more complicated than that. I'm sure theres some sort of formula you need to plug in

Jmf

Sorry, but that's just not true. There is current through D2 and D1, since the 10mA current must be shared between D1 and D2, and thus there is a voltage drop across both D1 and D2.

It's not an easy problem, actually, since there are two nonlinear elements. You can write down a few different equations though:

The diode equation (Shockley's) for D1 and D2.
Ohm's law across the resistor.
Kirchoff's current law at the node between the diodes and the current source.
Kirchoff's voltage law around the loop at bottom (that goes from ground, through the diodes and resistor back to ground).

That's 5 equations, though 3 of them are quite trivial, and after some substitution you can solve for R given V = 80mV (I believe).