Author Topic: Ebers-Moll and how to use it to calculate emitter current.  (Read 384 times)

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Offline dsharp02

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Ebers-Moll and how to use it to calculate emitter current.
« on: June 29, 2018, 01:02:28 pm »
I haven't had a lot of time to do any electronics for the past couple of years, so I've been re-reading TAoE.  How do you use the Ebers-Moll model to calculate the current for a given Vbe?  Wouldn't you have to know Ies?  How do you calculate Ies?

Sorry if this is a stupid question.

Thanks,
Dave
 

Offline kosine

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Re: Ebers-Moll and how to use it to calculate emitter current.
« Reply #1 on: June 29, 2018, 02:11:07 pm »
Don't have my copy of AoE to hand, but I recall the presented model is the simplified approximation, which is based on the idealised Shockley diode equation. Start by understanding that if you're keen to learn more.

The reverse saturation current (I_s) is a key parameter, but it can vary by quite a bit between actual transistors, and there are many other factors that affect the operating currents. As a result, Spice software tends to use the Gummel-Poon model to give a better approximation, but that's a bit more complicated. (Looks up "Gummel plot" as well. They're a very good way to understand BJTs. Surprised Dave hasn't done a video on it yet...)

If you really want to go down the rabbit hole, I'd suggest getting a copy of "Integrated Electronics: Analog and Digital Circuits and Systems" by Millman and Halkias. Secondhand copies are cheap, and it covers all the theory that Horowitz and Hill gloss over.
 

Offline Wimberleytech

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Re: Ebers-Moll and how to use it to calculate emitter current.
« Reply #2 on: June 29, 2018, 05:41:43 pm »
I haven't had a lot of time to do any electronics for the past couple of years, so I've been re-reading TAoE.  How do you use the Ebers-Moll model to calculate the current for a given Vbe?  Wouldn't you have to know Ies?  How do you calculate Ies?

Sorry if this is a stupid question.

Thanks,
Dave

 

Offline LvW

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Re: Ebers-Moll and how to use it to calculate emitter current.
« Reply #3 on: June 29, 2018, 07:00:10 pm »
Yes - Ebers-Moll is the correct relation between Vbe and Ie.
However - in practice, it plays no major role.
Why not? Because we do not know the value of Is. In addition, the current Is is strongly temperature dependent.
So - what to do?
The answer is: negative feedback.
It is common practice to bias the base with an suitable voltage and to implemet - in addition - negative DC feedback (in most cases we are using an emitter resitor RE).
This negative DC feedback remarkably reduces the sensitivity of the whole stage against uncertainties (tolerances) of the active device.
In principle, we have the same situation (and we do the same: negative DC feedback) for all opamp stages.
 


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