# Calculating Q points for diodes

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1. Feb 17, 2017

### whatphysics

1. The problem statement, all variables and given/known data
In Figure Q22, the empirical junction diode equation is : (eqn is in the picture attached)
for both diodes D1 and D2, where VT = 26 mV at room temperature, n = 1, IS = 3.5 x 10-14 A, R1 = 600 Ω, R2 = 300 Ω, R3 = 200 Ω, VS = 9 V. Calculate the Q-points ID = _____ mA and VD = _____ V of diode D1.

2. Relevant equations
Shockley Diode Equation?

3. The attempt at a solution
I tried to take Id=Vs-Vd/R3 and Vd=0.026ln(Id/Is)
I would try to sub in value of Vd=0.7V into Id eqn to get Id value and then beginning the iteration process and continue this until I get a non changing Id and Vd values to 3 d.p. However, I didn't get the right answers.
Was I wrong to take R3 only for the Id equation?

The right answers are 4.18mA and 0.66V
The answers I got are way off 41.39mA and 0.72V.

Any help is greatly appreciated, thank you!!

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2. Feb 17, 2017

### CWatters

That's not correct. There will be a voltage drop across R1.

3. Feb 17, 2017

### whatphysics

So should it be Id=Vs-Vd/(R3+R1) since R3 and R1 are in series?

4. Feb 17, 2017

### cnh1995

They are not.

Use node voltage analysis. You will get an equation of a straight line in terms of Vd and Id, and an exponential (or logarithmic) equation in terms of Vd and Id. You need to solve these two equations to find Vd and Id.

How do you solve this transcendental equation?

Last edited: Feb 17, 2017
5. Feb 17, 2017

### Staff: Mentor

Hint: Thevenin to reduce the source and resistor network first. Then it's a classic load-line situation.

6. Feb 17, 2017

### CWatters

+1

Unfortunately it's more complicated than that because of R2.

See the replies above for the way forward.