steveb,

Again, I don't understand your logic. You can't assume that only Vbe changes. If Vbe changes, then Ib changes too.

Certainly, Ib does change because Vbe makes it change. Sedra and Smith shows that. Vbe also also makes Ic change too. Ib does not control Ic, but it is an indicator of what Ic is, because both Ic and Ib are controlled by a common parameter which is Vbe.

If I use a current source to drive a base emitter junction in a BJT, I force the development of Vbe on the diode section. The Vbe then forces the Ic to develop. OK, Vbe seems to be the physically important control from the physics point of view. So what? The total device has still been controlled by current.

From a semiconductor physics point of view, Vbe controls Ic. Ib can be any value you want within the active region, but Vbe is really what is making Ic hold to its value. That does not mean that useful models and design cannot be made and done by taking advantage of the proportionality of Ib vs Ic. Sedra and Smith show no equation that relates Ic to Ib as it does Ic to Vbe.

Ok, I read through these. Yes, it is interesting but I see nothing new there.

The object of that reference was not to introduce new material, but to affirm what was already avered. It shows that I am not alone in my views, and that they are not some off the wall insight that was concocted just to cause controversy.

Analogy: If the owner of a company orders his manager to tell a worker to do something, and the manager tells the worker to do that thing, and the worker then goes and does that thing, who is in control?

The worker, the lowest person in the chain of command. He is the worker (Vbe) who directly affects what is going one. Notice the word "directly".

I'm not sure if this web-page has been referenced already, but I didn't see it. Again, I see the chicken and egg argument.

http://en.wikipedia.org/wiki/Talk:Bi...ion_transistor

Thank you very much. I never came across that reference before. I can't imagine how I missed it. I am going to quote two paragraphs from it which summarize my views about this subject. Just disregard if you already assimilated this material.

"Math models obscure the question as they simplify the concepts. Look to the full-blown details in order to understand how transistors actually work. As with any diode junction, the B-E junction contains an insulating "depletion layer" whose thickness is controlled by applied voltage. Make this insulating layer thicker, and the diode turns off (reverse biased), or make the layer thin enough, and charges are able to get across; the diode turns on. E.g. the diodes's current is controlled by the voltage applied to the diode's terminals. Now in a BJT there essentially are two currents in the same B-E diode junction: the base current and the Collector current, and... both are controlled by the thickness of the insulating B-E depletion layer. And the thickness of the B-E depletion layer is controlled by the applied voltage Vbe. THEREFORE, both the collector current Ic and the base current Ib are controlled by the Base-Emitter voltage Vbe. Things only SEEM confusing because Ib happens to be proportional to Ic. In reality Ib cannot affect Ic, instead both are affected by a third variable: the B-E voltage. Weird, eh? At the most fundamental level, BJTs and FETs are both controlled by voltage. However, in a BJT the main current must cross an insulating layer of variable thickness, while in an FET the main current remains in the conductive regions while the insulating regions encroach from the side. If a BJT acts like the dark lens in a pair of variable-density sunglasses, then an FET is more like a variable-aperature optical shutter or iris: a variable-sized empty hole. --Wjbeaty 21:29, Mar 20, 2005 (UTC) "

"It all boils down to whether we want to teach how transistors work, or whether we only want to solve some design equations. Never lose sight of the fact that the collector current is never directly controlled by the base current. If we drive the base with a constant current source, this creates a certain value of BE voltage. This BE voltage then determines the thickness of the depletion layer, which then controls the collector current. Yet if we could somehow hold Vbe constant while changing Ib, we'd find that Ib cannot control Ic. Be careful not to confuse the simplified "black box" mathematical model with the physics-based explanation of transistor behavior. Yes, simplified engineering equations say that Ic is determined by Ib, and these equations are incredibly useful... but these equations are "wrong" because they mislead us into believing that Ib directly affects Ic. It does not. Instead, Ib determies Vbe, and Vbe determines Ic, which shows that a transistor is a voltage-controlled device which pretends to be a current-controlled device. Many people seem to hate this fact, and try to talk themselves into believing that Ib directly affects Ic. But unfortunately there's no escaping the well-known transistor physics. If we want to know how transistors work, we have to confront the fact that the flow of Ib charges has no affect on the flow of Ic charges. Yet for simplified introductory classes where nobody cares how transistors actually work, it's fine to say that Ic is controlled by Ib... and then to never mention any complexities such as the fact that the control relies indirectly upon Vbe. On the other hand, if a more advanced textbook never mentions the Eber-Molls model, or worse, tries to use semiconductor physics concepts to "prove" that Ib affects Ic, then that author has a serious misconception, and is doing their readers a disservice. --Wjbeaty 20:02, 4 April 2006 (UTC)"

Ratch

 

Certainly, Ib does change because Vbe makes it change. Sedra and Smith shows that. Vbe also also makes Ic change too. Ib does not control Ic, but it is an indicator of what Ic is, because both Ic and Ib are controlled by a common parameter which is Vbe.

From a semiconductor physics point of view, Vbe controls Ic. Ib can be any value you want within the active region, but Vbe is really what is making Ic hold to its value. That does not mean that useful models and design cannot be made and done by taking advantage of the proportionality of Ib vs Ic. Sedra and Smith show no equation that relates Ic to Ib as it does Ic to Vbe.

Whilst you are placing a lot of faith in Sedra and Smith, can I get you to admit that Sedra and Smith also support the current-controlled model alongside the voltage-controlled model - Fifth Edition (the latest edition), Section 5.1 - Recapitulation and Equivalent-Circuit Models - equivalent circuits also given in figures 5.5(a) and 5.5(b).

A simple "yes" or "no" will suffice.

Dave

 

Analogy from steveb: If the owner of a company orders his manager to tell a worker to do something, and the manager tells the worker to do that thing, and the worker then goes and does that thing, who is in control?

Answer from Ratch: The worker, the lowest person in the chain of command. He is the worker (Vbe) who directly affects what is going one. Notice the word "directly".

Ok, I'm glad I put forth this analogy because this gets to the heart of our difference of opinion. In my mind, the owner of the company is in control. We each seem to operate with a different definition of control. That's fine. Now I understand your point of view.

 

Dave,

Whilst you are placing a lot of faith in Sedra and Smith, can I get you to admit that Sedra and Smith also support the current-controlled model alongside the voltage-controlled model - Fifth Edition (the latest edition), Section 5.1 - Recapitulation and Equivalent-Circuit Models - equivalent circuits also given in figures 5.5(a) and 5.5(b).

Certainly you can. Current control models are especially useful in BJT design. It is not even necessary to know that the Ib current is a indirect secondary method of control, and that Vbe is primary control to which Ic is really responding. I never was down on useful design models, only the false principle which states that BJT's are controlled directly by Ib current.

Ratch

 

Dave,



Certainly you can. Current control models are especially useful in BJT design. It is not even necessary to know that the Ib current is a indirect secondary method of control, and that Vbe is primary control to which Ic is really responding. I never was down on useful design models, only the false principle which states that BJT's are controlled directly by Ib current.

Ratch

So this does basically boil-down to definitions of what constitutes control (as if we didn't suspect that was the case ).

160+ posts across two threads...

Dave

 

Dave,

So this does basically boil-down to definitions of what constitutes control (as if we didn't suspect that was the case ).

160+ posts across two threads...

It would appear so.

Ratch

 

You know, you would have a lot more credibility if you ever showed something you built, designed from scratch, or improved on. I can't recall seeing one item.

Theory is one thing, but without applications it is usless. I apply transistor current theory with good results. Models don't have to be perfect, or even comprehensive to be useful. This is going to be the only time I post to this thread, I don't enjoy arguement for it's own sake. I like learning but it would be nice if it were useful, where are the practical applications?

 

steveb, please find attached the basics of the current-control model of the BJT.

Dave,

Thanks for posting this for me!

Steve

 

Bill Marsden,

You know, you would have a lot more credibility if you ever showed something you built, designed from scratch, or improved on. I can't recall seeing one item.

OK, here is a linear power supply I built back in 1973. I got the basic plans and idea from a magazine article, and I made many improvements to the basic design. Notice in the back the two heavy duty transistors and the big power resistor which is mounted outside where it can cool. This makes a pass transistor unit. One transistor is in series, and the other is in parallel with the resistor. This allows the power resistor to take most the hell from the power dissipation. Notice too all the strapping points on the bottom of the back. I could connect a similar power supply (PS) to it, and slave the voltage of the second PS to the first PS potentiometer. Other tricks involving two or more PS's can also be done. Also the sense leads can be made external by disconnecting from the strapping terminal the internal voltage, and bringing the two sense leads to the external load.

The front shows two meters for current and voltage, two indicator lights that show whether it is in voltage or current limiting, The controls for the voltage and current limiting are each ten turn pots. By the way, the current range switch is double ganged so that the voltmeter has its own switch. This keeps the voltage from being read across the shunt resistor which will cause inaccuracy for heavy current.

I wish I could show you more including the schematics, and especially the pass transistor circuit, but my zip file limit is 2 megs. If anyone wants to see more, then I can post further information.

I don't think this project increases my credibility at all. Theory stands on its own no matter what the practical application.

Ratch

 

I am worried that Dave may have slipped up on some tof the signs in his Kirchoff analysis, but haven't checked thoroughly so apologies in advance if I'm wrong.

Please look also at this thread

http://forum.allaboutcircuits.com/showthread.php?p=108872#post108872

And ask yourselves what level of explanation is appropriate for students at this level? (no disrespect is meant to the students, we all had to start somewhere)

Perhaps there should be two or even more levels of explanation in the textbook
Beginner - intermediate - advanced etc.

Perhaps also we could move discussion about the nature of BJTs and resistance from the thread on holes here. I have comments I would like to make on holes, but we seem to keep getting diverted in lots of threads to BJT current v voltage.

Finally the words 'fundamental control mechanism' don't only imply that the mechanism can cause something to happen. It equally implies also that he mechanism can prevent something happening.

Vbe manipulation is totally incapable of preventing irradiation/illumination causing transistor action.

Going back to my tap analogy, the photons act directly on the flow of charge carriers through the transistor.

So really for any mechanism to qualify as a 'fundamental control mechanism' it must be able to do this.

 

Perhaps, we shouldn't go back and forth like this. I'm going to go back to the full Ebers-Moll equations and rigorously derive the voltage and current control models and try to objectively see your point of view. It's been 20 years since I've done this, and I had a different mind-set back then. If I can reveal anything interesting, I'll post it.

We seem to have resolved the voltage vs. current debate. Or, at least I have in my own mind.

I thought I would post this current control model derived from the Ebers-Moll voltage control model. Dave was kind enough to post most of the work, but years ago I had put the results in a form that I preferred. Basically, I extended what Dave presented and explicitly show the effective current multiplier for a current control model.

The first boxed equation for the effective beta provides a complete model just as useful as the Ebers-Moll voltage control model. This equation does not assume perfect linearity as is often done with simple current control models. However, this equation ends up being more linear and sometimes easier to use than the voltage controlled model. The operating point (Vbc, Vbe and temperature) establish the effective current gain and any small signal deviations around that operating point appear almost linear. This model is essentially the same as what Dave presented, but I just show the linear relationship more explicitly.

The second boxed equation for the effective beta is approximate, but more revealing for the significant effects. The equation shows that Vce and temperature have the most dominant effects on current gain once saturation effects come into play. Note that Vce and temperature are important.

Of course, the Ebers-Moll voltage control model (and hence this current control model) is an idealized model.

I present this simply in case others find it interesting or useful. I've never seen this second approximation in a book, but I expect others have done something similar before. If anyone knows a reference, please post it. I'm not trying to take credit, but want to show this revealing formula.

 

I would like to return to the question of how do we enhance the AAC book?

I have posted a beginning for the physics and chemistry of the situation in a new thread here

http://forum.allaboutcircuits.com/showthread.php?t=17686

For the electronics I observe that the following are basic maths and physics, not models.

A transistor is a three terminal device.
There are three possible terminal currents.
label these ib, ie, ic
There are three possible interterminal voltages.
label these
Vbe, Vcb, Vec.

From basic physical laws

ib + ie + ic = 0

Vbe + Vcb + Vec = 0

From basic maths we have six variables and two equations.
Therefore we have four independent variables and two dependent ones that can be determined from a knowledge of the others plus the two equations.


If we introduce models of the transistor action, such as Ebbers-Moll or others we can deduce further relationships between the variables to assist calculation.

The above is true for all models, but once we introduce a model, subsequent theory is only true for the model validity conditions.