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Laplace

Apr 4, 2010
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The forced base current from a current generator is a external input which does not physically affect the diffusion process directly like Vbe does.
Unless the transistor base is connected to an external input, the transistor is inoperative. If the base is driven by a voltage source, then Vbe controls the base current. If the base is driven by a current source, then the current source controls the base current and Vbe adjusts itself accordingly. There is no escaping this physical reality.
 

Ratch

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Unless the transistor base is connected to an external input, the transistor is inoperative. If the base is driven by a voltage source, then Vbe controls the base current. If the base is driven by a current source, then the current source controls the base current and Vbe adjusts itself accordingly. There is no escaping this physical reality.
You are neglecting to mention something. Vbe is the internal voltage that defines and controls what Ib and Ic will be, because voltage controls the current according BJT physics. A external current source by definition cannot be considered to be the internal control of a device. That would make control of the BJT dependent on the external circuitry, which it is not.

Ratch
 

Laplace

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Vbe is the internal voltage...
Not true. Vbe is the voltage which appears at the external base and emitter terminals. Vbe enters its useful zone only when current is forced to flow from base to emitter.
 

LvW

Apr 12, 2014
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"Many errors, of a truth, consist merely in the application of the wrong names of things." - Spinoza

I think, our discussion has reached a point where two terms need some more clarification - otherwise, misunderstandings cannot be avoided.

1.) What means "control"? I think, we have two alternatives:
Imagine that the BJT is within a black box and I only have access to the three terminals (C, B, E).
We will observe that a current "injection" into the B node (read point 2 for "injection") will result in an Ic increase. I think, in this case and without knowing what happens within the box, it is correct to say that the input current controls the output current (from the practical point of view).
However, with some knowledge about transistor physics we can be more precise and say "in fact, it is the voltage across the B-E path that controls the current Ic.".
Now - the question arises where this controlling voltage comes from (see next point).

2.) Does a current IB really "produce" a voltage drop VBE? I don`t think so.
I am aware that we all speak about "voltage drops" and "current injection" - but these terms are closely related to the question "do current sources exist ?". My answer is : NO.
Each technical device we call "current source" is in reality a voltage source with an internal source resistance that is much larger than the load resistance.
Basic law: No current without a driving voltage!
From this it follows that a so called "current source" feeding a current into a load is nothing else than a simple voltage driven resistive voltage divider.
Hence, it is not the current which "produces" a voltage at the load resistor. Instead, we have a current - determined by the sum of both resistances - and a voltage division according to the resistor ratios.
As a result - physically spoken, it is not correct to say that an "injected" current IB would "control" the voltage VBE.
____________________
I hope that some of these theoretical considerations can help to avoid misunderstandings while using the terms "control", "current injection", "current source".
 

LvW

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What is the interest in knowing the exact value of Vbe when that value will drift?

I think, for a fair and fruitful discussion we shouldn`t mix theoretical/physical aspects with practical and application oriented aspects.
Unfortunately, without measuring the device we do not know the exact VBE value - but surely we would appreciate such a knowledge.
For this reason, we only can use a kind of "guess" - but we always use a value of app. 0.7V for calculating the value for the resistor RB between Vcc and the base node, don`t we?
And because of this "uncertainty" we provide DC feedback.
The value of VBE is of fundamental importance (because it determines the value of Ic) - however, because of the large tolerances (incl. temp. drift) we are using design strategies which can reduce the influence of the exact VBE value on circuit performance.
 

Ratch

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Not true. Vbe is the voltage which appears at the external base and emitter terminals..
Yes, but those terminals are directly bonded to the internal semiconductor slabs. That is as close to the internals of a BJT as an applied voltage can get. Besides, BJT physics says that Ic and Ib are determined by Vbe.

Vbe enters its useful zone only when current is forced to flow from base to emitter.
No, base current is waste current that cannot be eliminated. Base current will be present when Vbe is applied, but it is an indicator of collector current, not a cause. BJT physics prove that.

Ratch
 

LvW

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Vbe enters its useful zone only when current is forced to flow from base to emitter.
Currrent always is "forced" by a voltage (resp. by a corresponding E-field). It is a common misconception that the voltage-current pair would be something like a chicken-egg problem.
 

Laplace

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However, with some knowledge about transistor physics we can be more precise and say "in fact, it is the voltage across the B-E path that controls the current Ic.".
But with the drift in Vbe it is possible that the same value of Vbe will lead to different values of Ic, and that different values of Vbe will lead to the same value of Ic. In fact the base current is a much better indicator of collector current because the relationship is more linear than the transconductance.
As a result - physically spoken, it is not correct to say that an "injected" current IB would "control" the voltage VBE.
Except that the base-emitter junction is not a resistor and does not obey any voltage divider rule.

Does the base-emitter junction behave as a forward biased diode or not? If it does, then for purposes of this discussion, we can neglect the fact that the junction is actually part of a transistor when considering the base-emitter I-V characteristics.
 

Laplace

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Besides, BJT physics says that Ic and Ib are determined by Vbe.
That's obviously the problem right there. In BJT physics the value of Vbe is important. But in electronics we never apply a voltage directly to the base of a transistor; we apply a bias current. The value of Vbe is not relevant, except for the extents of its drift range.
 

LvW

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But with the drift in Vbe it is possible that the same value of Vbe will lead to different values of Ic, and that different values of Vbe will lead to the same value of Ic. In fact the base current is a much better indicator of collector current because the relationship is more linear than the transconductance.
.
...and what about the large tolerances of B=IC/IB ? I think, because of this uncertainty, most common-emitter stages are designed with a voltage divider instead of "current injection" using only one large resistor RB.
More than that, I think, the linearity is not a problem at all. If we would know the exact exponential function of IC=f(VBE) we could, of course, exactly determine the desired operational point.

Except that the base-emitter junction is not a resistor and does not obey any voltage divider rule.
.
Is this a novel law? Up to now I was of the opinion that this rule, of course, can be applied for non-linear resistors..(sorry - if this sounds a bit ironic).
How do you calculate the value of the resistor RB between Vcc and the base node (Vcc=IB*RB+VBE) ? Is this not a voltage division?

EDIT: Laplace, perhaps you refer to the voltage divider rule in its resistor form only [V2=VoR2/(R1+R2)].
My interpretation is simply that a voltage Vo is divided between two resistors according to their idividual values - that`s what I mean: V2/V1=R2/R1.

Does the base-emitter junction behave as a forward biased diode or not? If it does, then for purposes of this discussion, we can neglect the fact that the junction is actually part of a transistor when considering the base-emitter I-V characteristics.
Yes - if it helps?
 
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Ratch

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That's obviously the problem right there. In BJT physics the value of Vbe is important. But in electronics we never apply a voltage directly to the base of a transistor; we apply a bias current. The value of Vbe is not relevant, except for the extents of its drift range.

Yes, in BJT design, we try to minimize the nonlinear response of Vbe through judicious use of external components. I was not referring to design, but instead what controls the BJT internally as a device alone. By the way, drift current if very minimal in a forward biased diode as compared to the diffusion current.

Ratch
 

LvW

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In BJT physics the value of Vbe is important. But in electronics we never apply a voltage directly to the base of a transistor; we apply a bias current..
Do we really? In contrast, I am sure, most common-emitter stages rely on applying "a voltage directly to the base of a transistor" with the help of a low-resistive voltage divider.
I am aware that this is not an "ideal" voltage source because of known other constraints (signal input resistance). However, we always try to find a trade-off between applying a stiff voltage which - at the same time - still allows an acceptable input resistance (rule of thumb: base current app. 10% of the total current through the divider).
And think of the common-base configuration. Here we do not have the input resistance problem - and we would, of course, use a true voltage source at the base node. However, due to practical reasons we also use a voltage divider (and need only one single supply voltage source).
 

Laplace

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Do we really? In contrast, I am sure, most common-emitter stages rely on applying "a voltage directly to the base of a transistor" with the help of a low-resistive voltage divider.
Most common-emitter stages also employ an emitter resistor for negative voltage feedback. So in a real circuit the value of Vbe is not relevant (assumed to be a constant forward biased diode drop) nor is the value of Ib relevant (except that the high current gain of the transistor makes Ib small in relation the the current flowing in the bias resistors).
 

LvW

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Most common-emitter stages also employ an emitter resistor for negative voltage feedback. So in a real circuit the value of Vbe is not relevant (assumed to be a constant forward biased diode drop) nor is the value of Ib relevant (except that the high current gain of the transistor makes Ib small in relation the the current flowing in the bias resistors).
Laplace - with all respect, I hope you know that this is not a "serious" counter argument. We speak about principles of operation - and you are answering with a design strategy that is applied with the aim to cope with non-idealities (tolerances, temp. influence).
On the other hand, your answer is a very good argument in favor of biasing the base node with a "stiff" voltage: Negative voltage feedback via RE does only work for biasing with a voltage!
I am really surprised how you can come to the conclusion (post#49) that "we never apply a voltage directly to the base of a transistor; we apply a bias current".
 

Laplace

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I am really surprised how you can come to the conclusion (post#49) that "we never apply a voltage directly to the base of a transistor; we apply a bias current".
You are correct. That should have said, "we never apply a voltage directly to the base-emitter junction of a transistor..."
 

LvW

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I wrote: I am really surprised how you can come to the conclusion (post#49) that "we never apply a voltage directly to the base of a transistor; we apply a bias current".
Your answer: You are correct. That should have said, "we never apply a voltage directly to the base-emitter junction of a transistor..."

Again - with all respect: Is THAT your answer? Are you kidding me?
 

Laplace

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Are you kidding me?
What seems to be your problem? It is well known that that the I-V characteristic of a diode junction is much too temperature sensitive for any particular voltage to be applied. That's why Vbe is not relevant.
 

LvW

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What seems to be your problem? It is well known that that the I-V characteristic of a diode junction is much too temperature sensitive for any particular voltage to be applied. That's why Vbe is not relevant.
Laplace - I can assure you that I don`t have any "problems" regarding BJT working principles.
But do you remember that in one of your former posts you wrote: "Also note that while the BJT operates as a transconductance device (collector current controlled by base voltage),...."
And now you write: "Vbe is not relevant".
More than that, as I have mentioned already (my post#45) - you continuously are mixing theoretical/physical aspects with practical and application oriented aspects.
It`s regrettable, we turn around in circles. I think, I should leave this "discussion" now.
 
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Laplace

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It`s regrettable, we turn around in circles. I think, I should leave this "discussion" now.
If you have difficulty accepting the fact that Vbe controls the collector current, but that the particular value of Vbe is not relevant, then perhaps you should re-examine your understanding of transistor principles.
 

LvW

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Vbe controls the collector current, but that the particular value of Vbe is not relevant, then perhaps you should re-examine your understanding of transistor principles.
It remains your secret how you can explain this contradiction to somebody else.

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