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Osnel Jr
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- I know that the base to emitter current is what makes the transistor essentially turn on but why doesn't that deplete the second barrier and only the first
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Barrie Gilbert had little formal tech education beyond high school. He went far with what little he knew but his explanations are sheer nonsense.LvW said:Do not overestimate the role of the base current.
(In this context, one of the worlds best known D&D engineers - the late Barrie Gilbert - has used the term "defect" and "nuisance" for the base current).
Without any doubt , there is always a base current (which can be accounted for during the design of BJT-circuits), however, it is only the base-emitter voltage Vbe which determines the collector current (Shockleys famous equation).
This can be (and was) demonstrated and prooved.
(There is not a single proof or explanation for current control)
Parasitic? That means it does nothing. Base current can be useful.LvW said:Hi Claude...You will remember some of our earlier discussions on this matter.
I agree that - as you wrote - emitter current controls collector current per Ic = alpha*Ie.
No doubt about this.
And therefore (as you have stated): Ic = Ies*(exp(Vbe/Vt)+1).
Final conclusion: Vbe determines (controls) Ic.
The base current Ib plays no major role - as far as the voltage gain of amplifier stages is concerned. The existence of Ib just reduces the input resistance of BJT based circuits.
(As THIS was the original question).
Do you agree?
(By the way: According to my knowledge, Barrie Gilbert has "never said many times that base current should be zero"). His position was that Ib does not "control" (determine) Ic in a cause-effect-sense. And therefore, this current would be more or less parasitic.)
At last we have narrowed it down to two possible answers to the question of which came first, the chicken or the egg. We can simulate a chicken laying an egg, or we can simulate an egg hatching into a chicken. A simulation can show anything we want, depending on the order and direction of computation.cabraham said:Many blindly assume that the voltage determines or "controls" the current. But the simulation shows that voltage on a p-n junction changes as a result of a current change. The current goes from existing value to the new value quickly. Voltage slowly catches up.
Yes - I fully agree. Simulation is mathematics, nothing else. Our simulation programs can never reveal if the effect A is the cause or the result of the effect B. These simulations are based on relations only (forward/backwards).Baluncore said:At last we have narrowed it down to two possible answers to the question of which came first, the chicken or the egg. We can simulate a chicken laying an egg, or we can simulate an egg hatching into a chicken. A simulation can show anything we want, depending on the order and direction of computation.
OK - perhaps I have used a term ("parasitic") which is not correct...or it must be defined before.cabraham said:Parasitic? That means it does nothing. Base current can be useful...
Vbe is a loss, or parasite....
Hence one can view Ib as a parasite, since it doesn't power the load. But the same can be said for Vbe. Both can be called parasitic.
I do not intend to place a comment to this surprising claim.cabraham said:Many blindly assume that the voltage determines or "controls" the current. But the simulation shows that voltage on a p-n junction changes as a result of a current change. The current goes from existing value to the new value quickly. Voltage slowly catches up.
Diode current is NOT "controlled" by diode voltage.
Very good point! The l-V relation pretty much is chickens & eggs. I've said that for years.Baluncore said:At last we have narrowed it down to two possible answers to the question of which came first, the chicken or the egg. We can simulate a chicken laying an egg, or we can simulate an egg hatching into a chicken. A simulation can show anything we want, depending on the order and direction of computation.
But Ib is externally applied as well. The external source powers resistors & the b-e junction. Vbe is a drop incurred by charges moving through the n & p material. You claim that Vbe is externally applied, which is only true if b-e is shorted directly across a voltage source, which is certain doom for the bjt.LvW said:OK - perhaps I have used a term ("parasitic") which is not correct...or it must be defined before.
To me - "parasitic" means: A property which is not intended and which has an unwanted influence on the desired circuit behaviour (like the parasitic capacitance of a circuit node).
In this sense - the voltage Vbe is certainly NOT a parasitic one because it is a voltage which is externally applied (and not the result of an unwanted effect).
"...charges moving..."cabraham said:But Ib is externally applied as well. The external source powers resistors & the b-e junction. Vbe is a drop incurred by charges moving through the n & p material. You claim that Vbe is externally applied, which is only true if b-e is shorted directly across a voltage source, which is certain doom for the bjt.
The signal generator. Could be an antenna, phone cartridge, microphone, etc.LvW said:"...charges moving..."
May I simply ask - which force causes the charges to move?
OK - when it is not the E-field (caused by the applied voltage) which is responsible for the movement of charges (because you say that the E-field is "set up" by the moved charges), I must ask again ( as in my post#15):cabraham said:And how are E fields set up inside conducting material?
By transporting charges, of course.
Electric force does that. Singer Sue outputs acoustic energy that mic transduces into E field. This sets charges in motion. Charges move through mic cable & reach bjt b-e junction. Ib, Vbe, & Ie get changed from these new charges showing up. Any elaboration still needed.LvW said:OK - when it is not the E-field (caused by the applied voltage) which is responsible for the movement of charges (because you say that the E-field is "set up" by the moved charges), I must ask again ( as in my post#15):
Which force causes the charges to move?
OK - from the beginning, this was my conviction, of course.cabraham said:Electric force does that. Singer Sue outputs acoustic energy that mic transduces into E field.
I think you are trying too hard to apply cause and effect, first comes X then Y. The relationship V=I/R is circular. (Even when R is nonlinear and non-constant, it's still circular.)LvW said:Either Vbe is a drop - caused by moved charges; or it provides the electric force (E-field) which causes the charges to move.
Sorry, I cannot agree to that.anorlunda said:I think you are trying too hard to apply cause and effect, first comes X then Y. Voltage causes the current and current causes voltage.
You said that before. It's not true. Consider an inductor L with an initial current I0. Now we switch the inductor to disconnect it from the charging circuit, and put in in parallel with a resistance R or a Diode D. From the R's view, a current suddenly appeared which causes a voltage to appear across the R or D device.LvW said:However, asking which comes first (as a cause of the effect) it is always the voltage which comes "first". No current without a driving E-field within a conducting material.
As I have mentioned already before : Having such a discussion in a written form is problematic and can cause misunderstandings.anorlunda said:Consider a simple solar cell. Depending on the operating point, it can act like a current source (I versus V nearly horizontal) or like a voltage source (I versus V nearly vertical).
Believe me, you don't want to dig down to the fundamental physics of electricity which is QED. But if you're talking circuit analysis, which I believe we are, then the relationship is circular. Circuit analysis is compatible with physics as long as certain assumptions are valid. You can call it either simplified physics or computational tricks that are not physics; the difference is semantic.LvW said:In physics, I think, every effect has its cause (source). Do you think that the pair voltage/current is the only exception to this rule?
LvW said:Either Vbe is a drop - caused by moved charges; or it provides the electric force (E-field) which causes the charges to move.
Is it - in your eyes - really a "semantic debate" when we discuss the nature of an electrical energy source?anorlunda said:I won't engage in a meaningless semantic debate. If you insist that every current source is really a voltage source go ahead. But your either-or is still false.
But the "driving E field" is your assumption. How does a battery produce an E filed & voltage. The chemical process, redox, propels positive ions towards positive terminal, & negative ions towards neg terminal.LvW said:Sorry, I cannot agree to that.
Current causes voltage?
Lets assume we look at a simple resistor.
When the current through this resistor can produce the voltage across this device - which force drives resp. allows the current ? Current is identical to movement of charges - without an E-field there can be no continuous flow of charges.
(I speak about the common understanding of the quantity "current" within an electrical circuit resp. within parts like resistors or semiconductors; I do not refer to movement of charges caused by a chemical process or by diffusion/drift effects.)
Of course, during calculations and analysis of electrical circuits we can ASSUME that according to V=I*R the current I could "produce" the voltage V - from the math point of view this works.
However, asking which comes first (as a cause of the effect) it is always the voltage which comes "first". No current without a driving E-field within a conducting material.
anorlunda said:Believe me, you don't want to dig down to the fundamental physics of electricity which is QED. But if you're talking circuit analysis, which I believe we are, then the relationship is circular. Circuit analysis is compatible with physics as long as certain assumptions are valid. You can call it either simplified physics or computational tricks that are not physics; the difference is semantic.
I won't engage in a meaningless semantic debate. If you insist that every current source is really a voltage source go ahead. But your either-or is still false.
Vbe does NOT "modulate height of barrier". Also how can diffusion "move the charges"? Diffusion is the tendency for charges to spread out until all regions have equal concentration.eq1 said:This thread kind went off topic but the answer to the original question is: charge doesn't move from C to B because that would be uphill in energy when the NPN is in active mode. [1] Why is the collector at the lowest energy? Because that's how the NPN is constructed. We dope the collector that way on purpose.
I know textbooks like to draw BJTs like back to back diodes but a BJT is not a back to back diode. (I suppose it is that, but it is also more.) If you want proof try the following experiment. Wire two diodes back to back on a bread board, see if you get a transistor. When you understand why that didn't work you'll understand BJTs.
Personally, I like to think of the BJT as voltage controlled. V(B,E) modules the height of the barrier from E-B. When the barrier is low enough charge from E can flow into B and then it's all downhill from there. Personally, I am not a fan of the water analogies but it's kinda like the lowering of a flood gate in a dam, once the gate gets below the water line, things start to move. In a dam the force moving the water is gravity. In a BJT its diffusion moving the charge. And it kinda shows how the amplification works, i.e. a small movement of the gate height leads to a large change in the amount of water in the river below. And I suppose it can even have some implications on this discussion. i.e. for the dam, which controls the movement of water? The flood gate or gravity? And of course, for the case of the dam, the answer is both. But the analogy is getting a bit stretched.
My $0.02 anyway.
[1] https://en.wikipedia.org/wiki/File:Bjt_forward_active_bands.svg
In your mind, what is the difference between "move" and "spread out"?cabraham said:Also how can diffusion "move the charges"? Diffusion is the tendency for charges to spread out until all regions have equal concentration.
You’re wielding Shockley’s equation like it’s an axiom but it’s not. It’s a solution to a very particular system known as a BJT. But, where did Shockley derive it from? And what experimental evidence supports those equations? And what do they have to say about this topic? I would write it down but Dr. Hu has done an excellent job for me and I doubt I can improve upon his work. In addition Berkeley has made his work available for free! I suggest you take advantage of it.cabraham said:But Shockley's equation is just as easily expressed as follows:
Vbe = Vt*ln((Ie/Ies)+1).
I realize SE (Shockley equation) is NOT an axiom. In semiconductor physics class,cwe derived it as homework.eq1 said:You’re wielding Shockley’s equation like it’s an axiom but it’s not. It’s a solution to a very particular system known as a BJT. But, where did Shockley derive it from? And what experimental evidence supports those equations? And what do they have to say about this topic? I would write it down but Dr. Hu has done an excellent job for me and I doubt I can improve upon his work. In addition Berkeley has made his work available for free! I suggest you take advantage of it.
Edit: I also just remembered Sedra & Smith has a pretty decent derivation of the Shockley equation (and MOSFET and PN junction for that matter) just using dimensional analysis which might be more approachable. It's definitely not free but it's been a standard microelectronics text for decades so I think most libraries, pretty much anywhere in the world, should be able to obtain it. The problem is that derivation will only work for a 1d transistor, basically one line in the cross section. But it illustrates the point nicely and succinctly.
A transistor is a semiconductor device that is used to amplify or switch electronic signals. It is made up of three layers of material - a collector, a base, and an emitter - and works by controlling the flow of current between the collector and emitter using a smaller current at the base.
A transistor works by using a small current at the base to control the flow of a larger current between the collector and emitter. The base current activates the transistor, allowing current to flow from the collector to the emitter. The amount of current at the base determines the amount of current that flows through the transistor, making it an effective amplifier or switch.
The base current is the small current that is used to control the larger current between the collector and emitter in a transistor. It is typically a fraction of the collector current and is used to switch the transistor on and off or to amplify the signal.
The base current is crucial in determining the behavior of a transistor. It controls the amount of current that flows through the transistor and can be used to switch the transistor on and off or to amplify the signal. By varying the base current, the overall performance of the transistor can be controlled.
The collector current is directly proportional to the base current in a transistor. This means that as the base current increases, the collector current also increases. However, the collector current is typically much larger than the base current, making the base current an effective control mechanism for the transistor.