Today`s article about loadline curve

Thread Starter

Janis59

Joined Aug 21, 2017
1,148
https://www.allaboutcircuits.com/technical-articles/how-is-a-load-line-used-in-circuit-design/

The method explained I am using several years telling for schoolboys and schoolgirls the basic concepts of semiconductor biasing, when physics program touches the basics of electronics, but as the engineer I had never used to use this technique - simply not exist any diodes having that Uf=f(if) curve at datasheet except a very raritary minority from very the time apart. So I am telling them, this is concept only; with very difficult implementation but giving a sound view on the principle.

Therefore I would vote for attaching the similar sentence even in this article above. Your mind??
 

MrChips

Joined Oct 2, 2009
20,882
Finding the Q-point in a circuit requires solving two simultaneous equations.
One equation is the equation of a device that is often non-linear.
The second equation is usually that of a resistive load which is linear.

The second equation is easily derived from circuit analysis.

The first equation is more difficult to come by since it depends on semiconductior physics and information provided in manufacturer's datasheet which could be nonexistent.

So what to do?

It is perfectly ok to use a model that approximates the I-V characteristics of the nonlinear device.

Load line analysis is a simple graphical technique for solving the two simultaneous equations in a very intuitive manner.
If it works and is an effective teaching tool why not use it. I use it all the time for teaching.

1581950073274.png

BTW, the red I-V curve above was determined manually by measuring I over different supply V (with a series resistor) using a red LED.
 

Thread Starter

Janis59

Joined Aug 21, 2017
1,148
RE: ""Then, what real common diodes show?""
Of course the table it show
Example? Okay, please find me a u(f)=F from (i(f) graph for any diode in catalog https://www.radiolibrary.ru/reference/diod.html. Dont like, okay, take the catalog http://www.radio-komplekt.ru/component_ref.php?param=diodes
Dont like anyway? Okay, I agree that good firms makes any possible graphs (and then one pays for the "service", but such are not a majority. Most of it worldwide comes cheap, and then with some narrowly selected figures at certain current points instead of graph.

Happily, most of the applications it isnt very much important the precise value of voltage on diode. Enough is with approximate.
 

Thread Starter

Janis59

Joined Aug 21, 2017
1,148
RE: Wolframore - yes, this is the common way. It have a correct name the Schokley equation, or in case of bjt the Ebers-Moll equations. However there is one small ugly term labeled "an ideality factor" which never no-one is not showing up how large it is.
 

MrAl

Joined Jun 17, 2014
7,487
RE: Wolframore - yes, this is the common way. It have a correct name the Schokley equation, or in case of bjt the Ebers-Moll equations. However there is one small ugly term labeled "an ideality factor" which never no-one is not showing up how large it is.
Hi,

That factor is extracted from the physical transistor but can be found in a spice model for the transistor. It is often given as N or NF.
For example, a simple transistor using the least number of parameters for a transistor would have these three:
Is=1e-14
NF=1
BF=100
where Is is the saturation current, NF the forward ideality factor, BF the forward beta.
These are given in a spice model for a given transistor and may vary depending on the transistor.
 

Wolframore

Joined Jan 21, 2019
1,742
Doing some reading on η it seems that it’s a correction factor and one that can not be expressed as it has to do with the shape/flatness of the junction and bandgap. Suffice to say for silicon diodes it is about 1.1 or less and for BJT used as diodes about 1.01. η is always found experimentally. Some components can have more than 2 which would shift our curve quite a bit. For example for GaaN based LEDs η can be quite high. So it appears it’s a correction factor put into the Shockley equation to account for the differences and generally near 1 for silicon and so largely ignored. The factor is significant for germanium but it’s been a long time since I’ve seen those.

I believe our lab tonight we will be building exponential circuits using a BJT and if the results are not as calulated it will be from η. We should be able to identify the value as a corrections to kT/q which we approximate anyhow.
 
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