# Diode circuit question

#### deki

Joined Sep 6, 2011
23
Question: Using the diode equation: Just want to know whether I have the right idea: so to determine the reverse saturation current (Is), I just determine it for each value of Vd that was given? So finding Is for 0.6, 0.65, 0.7 etc?

The question seems to be divided into two parts, one that just asks for the reverse saturation current, and then to sketch a graph for points Vd=0.6, 0.65 etc. But it's not as if we're already given an initial Vd value to determine the reverse saturation current, so I'm guessing you just find it for the various values given?

Also, for plotting the load line part, I understand that I will be drawing my load line from 12V to an Id value. How do I determine this Id value? I'm guessing I need to find the line equation, but I'm not sure how to do that because of the diode. Do I just model the diode as a resistor in series with a voltage source? But then again, I haven't been given the Rd and Von value for this configuration..

Thanks.

#### t_n_k

Joined Mar 6, 2009
5,455
You'll have to find Vd first up.

As a start you'll notice

The current in R1, I1=(12-Vd)/2000

But

I1=I2+5.3mA

Where the current in R2 is I2=Vd/2000

Given this you can solve for Vd.

#### deki

Joined Sep 6, 2011
23
Thanks for your help. I thought of doing basic nodal analysis, but thought that I had to model the diode as a resistor in series with a voltage source.

#### t_n_k

Joined Mar 6, 2009
5,455
Thanks for your help. I thought of doing basic nodal analysis, but thought that I had to model the diode as a resistor in series with a voltage source.
It's more complicated than that - that's why you were given the Shockley diode equation.

#### deki

Joined Sep 6, 2011
23
So for the graph part (now that I have my reverse saturation current), I'd just want to find the individudual Id values corressponding to the given Vd values, right? Then plot Id vs Vd..

#### t_n_k

Joined Mar 6, 2009
5,455
Correct - what did you find for the value of Is?

#### deki

Joined Sep 6, 2011
23
4.14*10^-11 Amps (and Vd=0.7V)

Also, is my load line equation just the nodal analysis part: I1 = I2 + Id, and just solve for Id, then note down the values of Id and Vd, when either equals zero (for the vertical/horizontal intercepts)?

#### t_n_k

Joined Mar 6, 2009
5,455

You seem determined to use nodal analysis when there's no reason to do so.

Convert the DC source R1 and R2 to a Thevenin equivalent and draw the load line on the VI axes using Vth and Rth. Then plot the diode VI characteristic as deduced on the same axes and look for the point of intersection.

I just tried that and I ended up getting the same answers anyway If anything, at least I confirmed to myself that it's right. But you're right, I always jump into nodal analysis when there is a quicker/easier way to do it (such as Thevenin's theorem), and I keep on reminding myself that there are other ways, thanks!