Why is my solution for real diode analysis wrong?

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Homework Help Overview

The discussion revolves around the analysis of a real diode, specifically addressing a participant's incorrect solution related to the diode equation. The subject area includes semiconductor physics and circuit analysis.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the importance of the thermal voltage (VT) in the diode equation and question its treatment in the original poster's solution. There are inquiries about the approximation of VT at room temperature and its variability based on diode characteristics.

Discussion Status

Some participants have offered guidance regarding the need to consider the actual value of VT for the specific diode in question. There is an exploration of potential methods to approach the problem, including the suggestion to solve simultaneous equations based on provided circuit conditions.

Contextual Notes

Participants note that the original poster's solution may lack consideration of the specific characteristics of the diode, and there is an emphasis on the need for accurate values in the analysis.

theBEAST
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Homework Statement


9XUbpS3.png


Homework Equations


I=Ise(V/VT)

The Attempt at a Solution


The solution I got is wrong. Does anyone know why?

Here is my attempt, the image was too large for this forum so here is the link:
http://i.imgur.com/4tuGxdO.jpg
 
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You ignored VT in the exponent.

ehild
 
ehild said:
You ignored VT in the exponent.

ehild

Isn't the VT value the same (approximately 0.025V at room temperature)?
 
theBEAST said:
Isn't the VT value the same (approximately 0.025V at room temperature)?

The approximation is just that, an approximation. It is a good "typical" value for run-of-the-mill diodes. The actual value for a given diode depends upon the details of the diode doping. You should determine its true value for this problem.
 
theBEAST said:
Isn't the VT value the same (approximately 0.025V at room temperature)?

Even in case it is, I1/I2 = e(V1-V2)/VT

ehild
 
You may need to run a few simultaneous equations. Here's my suggestion: Use the generic information provided to solve for Is and VT before proceeding further. Then from there you can solve for R using the fact that both sides of the circuit must equate 10mA.
 

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