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Author Topic: Ebers Moll model of BJT  (Read 2986 times)
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RedBull
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« on: November 22, 2008, 02:18:49 14:18 »

I recently have been studying Ebers Moll model of BJT, and i have been unable to find answer of this particular question: Why is Ebers Moll model known as large-signal current model of BJT? and what is meant by large-signal model?
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Biggles
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« Reply #1 on: November 22, 2008, 03:41:19 15:41 »

If a model works for all signals, then it is called a large signal model.

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dennis78
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« Reply #2 on: November 22, 2008, 03:54:24 15:54 »

It isn't full answer,but: because  Ebers Moll model include nonlinearity's BJT (most of transfer functions BJT's have exponential character)-> mathematical equations in this model are valid in full range  singnals (over 90%). If we talk about full range model, its mean large signal model (as Biggles said).
I hope that this will help you. 
« Last Edit: November 22, 2008, 03:57:02 15:57 by dennis78 » Logged
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« Reply #3 on: November 22, 2008, 04:40:02 16:40 »

"The large-signal model is used to determine the DC operating point (VBE,  VCE, IB, IC) of the BJT. The small-signal model is used to determine how the output responds to an input signal."



"Use of the large-signal or small-signal model depends on the magnitude of the driving source. In applications where the driving currents or the driving voltages have large amplitudes, large-signal models are used. In circuits where the signal does not deviate much from the dc biasing point, small-signal models are more suitable. For DC conditions and very-low-frequency applications, DC equivalent circuit models are used."
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dennis78
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« Reply #4 on: November 22, 2008, 05:17:38 17:17 »

"The large-signal model is used to determine the DC operating point (VBE,  VCE, IB, IC) of the BJT. The small-signal model is used to determine how the output responds to an input signal."



"Use of the large-signal or small-signal model depends on the magnitude of the driving source. In applications where the driving currents or the driving voltages have large amplitudes, large-signal models are used. In circuits where the signal does not deviate much from the dc biasing point, small-signal models are more suitable. For DC conditions and very-low-frequency applications, DC equivalent circuit models are used."

Yes, you are right, but in generaly, you can always use Ebers Moll model in analyse output/input (of course, it isn't many practice is most of cases - equations are very coplex to solvea). For example, audio power amplifier is analysed with model who like as E-B model  in many situations:when you find transfer function, factor of distorsion,... Result of linearisation (around DC operating point)  E-B model equations is linear equation for small-signal models.
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RedBull
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« Reply #5 on: November 22, 2008, 06:25:56 18:25 »

"Use of the large-signal or small-signal model depends on the magnitude of the driving source" Does this mean then that Ebers Moll model is both a large signal as well as small signal model depending on in which case is it being modelled?
also is Ebers Moll equations mainly used to determine DC operating point?
and large signal means the signal that shifts the optimum bias level of BJT. Correct me if i'm wrong.
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dennis78
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« Reply #6 on: November 22, 2008, 06:51:17 18:51 »

It isn't very simple to explain because don't exist clear boundaries. E-B model is full, general model. Others are simple ways of this model. If signals on some terminals BJT's changes in long range, effect of nonlinearity BJT is increasing. In that situation linear equations(H parameters) isn't valid. You mus use EB model. When signal change  is small, nonlinearity effetcs are minor and you dont need EB.
You don't nedd EB model in determining DC bias point if you use equations type ic=f(ib) (you always write ic=beta*ib) while solving el. circuits, because it dependency is linear in long range. If you use ic=f(ube), you need EB, but i dont see reason for this equation in determinig DC bias in 99% cases.


   
« Last Edit: November 22, 2008, 07:04:26 19:04 by dennis78 » Logged
RedBull
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« Reply #7 on: November 22, 2008, 07:42:05 19:42 »

It isn't very simple to explain because don't exist clear boundaries. E-B model is full, general model. Others are simple ways of this model. If signals on some terminals BJT's changes in long range, effect of nonlinearity BJT is increasing. In that situation linear equations(H parameters) isn't valid. You mus use EB model. When signal change  is small, nonlinearity effetcs are minor and you dont need EB.
You don't nedd EB model in determining DC bias point if you use equations type ic=f(ib) (you always write ic=beta*ib) while solving el. circuits, because it dependency is linear in long range. If you use ic=f(ube), you need EB, but i dont see reason for this equation in determinig DC bias in 99% cases.


   
Oh...I'm beginning to understand. So Ebers Moll is to be applied when Ic is expressed non-linearly in terms of Vbe.
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