BJT transistors as voltage-controlled devices

Discussion in 'Feedback and Suggestions' started by tony_kuphaldt, Nov 28, 2008.

  1. tony_kuphaldt

    Thread Starter New Member

    Nov 28, 2008
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    Hi Everybody,

    I recently received an email from a reader suggesting a change to the chapter on bipolar junction transistors, citing a comment by Douglas Self on transistor theory: namely that BJT's are actually voltage-controlled devices and not current-controlled. Here is the email in its entirety:

    <email>

    Hi Tony,

    I had a quick look at your chapter on bipolar transistors and was
    very disappointed that you are perpetuating the misleading myth that
    transistors are current controlled devices - they are not! You are
    doing a disservice to education.

    A transistor is governed by the transistor equation that states that
    Ic is a function of Vbe. The fact that this equation holds for many
    decades of current should raise confidence that this is the actual
    governing equation.

    The fact that the collector and base currents differ by hfe (which
    varies a lot due to many factors) is merely a SIDE-EFFECT of
    transistor operation, it is not the BASIS of operation. Any design
    based on hfe is a bad design!

    I agree with Douglas Self's comments
    (http://www.dself.dsl.pipex.com/ampins/discrete/discrete.htm):

    "There is one thing to get straight first. The transistor is a
    voltage-operated device. What counts is the base-emitter voltage, or
    Vbe. Certainly a BJT needs base current to flow for it to operate, but
    this is really an annoying imperfection rather than the basis of
    operation. I appreciate this may take some digesting; far too many
    textbooks say something like "a small current flowing into the base
    controls a much larger current flowing into the collector". There is
    no prize for locating the source of this quotation. In fact the only
    truly current-operated amplifying device that comes to mind is the
    Hall-effect multiplier, and you don't come across those every day.
    I've certainly never seen one."

    [I think Horowitz and Hill are the originators of this quotation!]

    Please rethink how you teach this!

    Best regards,
    Fred.

    </email>

    This is an issue that crossed my radar after I had "handed the baton" to the capable folks at AllAboutCircuits, but I never pursued it further. Perhaps it is something to add to the "to do" list for future versions of the book.

    Anyway, I'd like to say "thank you" to everyone participating in this project, especially Dennis Crunkilton. You have made my dream of an open-source, collaboratively-developed textbook a reality. Students at my college still use the book in its every-expanding form, and it stands as an example of the great things that can get done when motivated people pool their talents.

    I've started a new textbook project, this one aimed at a more specialized field: industrial instrumentation and automation. You can find this work-in-progress at:

    http://openbookproject.net/books/socratic/sinst/book/liii.pdf

    This time I've used the Creative Commons "Attribution" license instead of the DSL to copyleft the work. The CC Attribution license is more liberal than the DSL, merely asking for proper attribution of credit if my new work is modified, extended, etc. This is the same CC license I've used on my "Socratic" teaching worksheets.

    - Tony

    P.S.
    You might notice a different user name ("tony_kuphaldt" instead of "tonykuphaldt"). I lost my password to the old login, and the only email account recognized by the lost password utility was my ancient "liec0" Lycos account, which I've also lost the password for. So . . . I created a new account for these forums (and I'm saving this password in a safe place!). If the Admin needs to verify this, feel free to contact me at work:

    Tony Kuphaldt -- Instructor, Instrumentation and Control Technology
    Bellingham Technical College
    3028 Lindbergh Avenue
    Bellingham, WA 98225-1599
    (360)-752-8477 [office]
    (360)-752-7277 [fax]
    tkuphald@btc.ctc.edu
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    Even if the base contention is right (which I don't agree with) there are plenty of cases you teach a simpler concept before hitting students with advanced stuff. AM radio and sidebands come to mind.

    This is a text book, it is also a reference material. We are trying to get people through their electronics course with the best grades they can make. If they start going down this road they will be very confused indeed, and could fail some critical courses.

    If you feel strongly about this then you should do the rewrites. Just a suggestion, make it a separate chapter, and state it is advanced theory.

    We've been down this road before, with another poster named Ratch, and the discussion was pretty intense. It started with a new student asking basic questions about transistors for his course. Ratch has come up with many other threads, such as "Existence of Electricity Questioned" which has left us all a little sensitized. I don't argue, I come here for other reasons, so I just read and occasionally grit my teeth.
     
    Last edited: Nov 29, 2008
  3. steveb

    Senior Member

    Jul 3, 2008
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    469
    I would be the first one to say that a transistor is a voltage controlled device, if I had to choose one parameter. I mentioned this is another post myself. However, one could argue that the controlling parameter is really a conceptual idea.

    We say that Vbe is the controlling parameter if we are using the Ebers-Moll type model. However, this model also has temperature in it. So is it also a temperature controlled device? Well, typically no because we usually don't have control on the temperature and we design the circuit performance to be largely insensitive to temperature changes.

    Perhaps a better parameter to consider is the ratio Vbe/Vt, but who wants to do that? It's just more confusing to think about. Also, there is temperature sensitivity of the reverse saturation current, so it's still not accurate.

    We sometimes design a circuit with the idea of controlling current. For example a CE/CC topology with a large emitter resistor Re allows us to control the emitter current directly. So this would appear to be a current controlled transistor. Of course, someone else may say that it is still a Vbe controlled device, but the Re acts as a feedback resistor and provides control. They would be right. But, what about the case where the emitter current is directly driven with a current source. That sure appears to be current control, and it's certainly conceptually easier to think of it that way, even if not strictly true.

    My point is that the transistor is a very complex nonlinear device with more than one parameter controlling the operation. Also, there are different models one may use to understand it. So the idea of having one controlling parameter is not really true.

    I do think it is important to discuss these details in the introductory material because it is clear that far too many people think that a transistor is a simple current multiplier with a constant Hfe or beta.
     
  4. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    Tony, it is good to hear from you. The project has come along nicely since the "passing of the baton" some 4 years ago, and no doubt you will have noticed that AAC has embraced the Socractic Project: http://www.allaboutcircuits.com/worksheets/index.html

    As for the point you raise in you OP: there are few, if any, topics of discussion that have raised such a heated debate as this one (see here if you want to read the whole discussion). Naturally, this discussion did not give us a definitive answer, but allowed everyone to have input to what is a rather fundamental question.

    I am of the opinion that it is correct to describe the BJT as being both voltage and current controlled depending on how you model the BJT function; a notion supported by Sedra and Smith. And the inextricable link between Ib and Vbe means that it is nonsensical to disassociate these parameters in the way some do when describing the BJT as solely a voltage or current controlled device. Saying that if that link did not exist and the "an annoying imperfection" that is the base current did not exist, we would not have a BJT, but in essence a FET which is a voltage-controlled device.

    From an educational perspective, I think it is also reckless to describe BJTs using only one model; my experience tells me that the voltage-controlled model is better suited for academic and more analytical treatise of BJTs, whereas the current-controlled model is better suited to a practical treatise of BJTs.

    I don't disagree with the voltage-controlled model, I just think it is loose and an incomplete description of the BJT. Others may disagree.

    Dave
     
  5. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Again, we are seeing the stark contrast of pure science and applied science!

    I am willing to bet there are hundreds, if not thousands, of engineers out there that have successfully designed complex transistor circuits without knowing that the transistor is actually more accurately described as a 'voltage-controlled' device. The simplification of labeling it a current-controlled device has given them the ability to use the transistor(s) in a design without knowing all the nitty-gritty.

    On the other hand, those studying solid-state physics, would need to consider that it is a voltage-controlled device. They would need to understand the complex interactions, the quasi-fermi levels and what not. This is because a true and deep understanding is necessary to develop better technologies!

    Recently, I was told that Ohm's law was a simplified result of one of Maxwell's equations. Little did I know, there is more to it if I wanted there to be! Nonetheless, I have designed many circuits from this foundation with no mysterious and unexplained phenomena that can only be explained by an ignored displacement current, etc. has occured!

    I guess this turned into a rant :(

    Steve
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I suppose this gets into a discussion as to the meaning and application of "control". I do not think Vbe is an adequate control mechanism for, say, an audio amplifier. I could be convinced of narrow-mindedness if I get a circuit that demonstrates Vbe control of a 2N3055 pushing 8 amps @60 volts into a speaker.

    Being a technician leads me to look for practical applications and explanations.
     
  7. Ratch

    New Member

    Mar 20, 2007
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  8. tony_kuphaldt

    Thread Starter New Member

    Nov 28, 2008
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    0
    If I had time to go back and revise sections of the "Lessons In Electric Circuits" book series, the first place I would start is the Semiconductors volume, specifically the chapters dealing with transistor theory. I will be the first to admit this section is over-simplified, even for a text aimed at entry-level students of electronics.

    There is a reason I decided to present the email from Fred rather than suggest he directly contact the forum himself. His opening paragraph -- where he accuses me of "doing a disservice to education" -- suggested a personality more interested in disputation than constructive dialogue. I figured if I presented the concept as a seed for discussion, there might be a better chance of a reasoned conversation than if Fred did the honors. However, what I seem to have stumbled upon is the scorched earth of a previous battle. My sincerest apologies for opening old wounds!

    What intrigued me mostly about Douglas Self's article was his claim that "every transistor obeys this [Shockley ideal diode equation] law with startling accuracy over nine or ten decades of Ic." I would like to see actual experimental data supporting this claim, and was hoping someone in the AAC forums might have access to such data. If it were not for the complex task of stabilizing junction temperature over nine or ten decades of current, I would be tempted to gather the data myself. Certainly, some experimental evidence would have brought much clarity to the "BJT's don't work like that" thread.

    Regarding the discussion on Ohm's Law, Ratch, I have already addressed this in the DC volume of the book series (http://www.allaboutcircuits.com/vol_1/chpt_2/6.html). As with the "Existence of electricity questioned" thread, the "Ohm's Law is not V=IR" thread seems to me mostly a matter of semantics, made all the worse by what appears to be a deliberately provocative title. There are more constructive ways to explore fundamental principles of electronic circuits than to argue over the labels applied to those principles.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    For what it's worth I like the book, and hope to contribute here and there. If you go to my AAC blog and check out The 555 Projects you will get a good sample of my work.

    Thank you for all your efforts.

    You ever need a graphic type just ask.
     
  10. Ratch

    New Member

    Mar 20, 2007
    1,068
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    tony_kuphaldt,

    I don't know about nine or ten decades, because it is an ideal equation. This link http://en.wikipedia.org/wiki/Diode says that it does not account for among other things, recombination-generation (R-G) current, photon-assisted R-G,and the “leveling off” of the I–V curve at high forward bias due to internal resistance. More terms would have to be added to this equation to account for secondary effects at extreme values. The point is that Vbe determines the current through a transistor no matter how it is biased. Unfortunately it is highly exponential, so that a Vbe of 0 to 1 volt or so will bring a transistor from cutoff into saturation or thereabouts. The "waste" base current is also exponential, so the collector current and the base current are proportional to each other, and can be (within a specific range) used to calculate each other if either is known. But conceptionally speaking, a BJT is a voltage-controlled device. If you read my link, you will see that I consulted with a professor of EE at the U of Colorado, who also said a BJT is voltage-controlled.

    We are talking about two separate issues here. 1)What is Ohm's law?; and 2) what does electricity mean?

    I and a few others aver that the formula V=IR is not Ohm's law, but is instead the definition of resistance. The real Ohm's law is The resistance of metallic conductor is the same no matter what applied voltage is used to measure it. In other words, a conductor obeys Ohm's law if its V-I curve is linear. I gave references for that statement in my previous link which you must have read. It included references to two good physics textbooks. Since that time I have perused other college level textbooks and they confirm what I said, although not with the clarity and directness of Resnick and Serway. So Ohm's law is a material property of electrical linearity, not a law of nature. Calling V=IR Ohm's law is not a matter semantics, it is a repeated misnomer.

    The other topic about what electricity is simply a statement that the word "electricity" has so many meanings that it is almost useless. It means energy, a field of study, an application of electrical energy, the existence of electrical energy, and on and on. That is obvious, and should not have generated the heat it did.

    Ratch
     
  11. tony_kuphaldt

    Thread Starter New Member

    Nov 28, 2008
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    0
    Ratch,

    I'm afraid there is little I can say with regard to transistor operation that hasn't already been said in the 13 pages of the "BJT's don't work like that" thread. If I were still developing the Lessons In Electric Circuits books, my next step would be to edit the Semiconductor volume to include the voltage-dependent characteristics of BJTs -- and as much of the underlying theory -- without being a distraction to readers who just want to figure out how to use the devices to build practical circuits. Unfortunately, I am not currently in a position to do that. Perhaps someone else is, and would like to take on this writing challenge.

    As for the "Ohm's Law is not V=IR" and the "Existence of electricity questioned" threads, the single unifying issue I see in both is disputation over the meaning of words. In the former thread, the dispute is over how we use the word "Law." Does a "Law" refer to a quantitative function broadly applicable over all possible conditions, without known exception? Given this definition, we must admit the existence of very few "Laws" indeed. A great many rules and principles in science called "Laws" (including Ohm's Law) hold true only for certain conditions, or are merely approximations (or limiting cases) of more general Laws. In the latter thread, the meaning (or rather non-meaning) of the word "electricity" is the center of debate. This is a purely semantic discussion, as clearly evidenced by the linguistic debate in later pages of the thread. In neither of these threads have I seen evidence of practical disagreement -- it's all about the labels applied to certain phenomena.

    Where I have a problem with *all* these threads is the combative tone. In each case, the thread opens with some form of indictment: common textbook presentations of some concept or another are wrong, and you are here to set the record straight. In each case, however, the issue is not so much a matter of someone being right or wrong as it is a matter of using imprecise terms and/or simplifying assumptions intended to expedite practical application. Unfortunately for the readers of these threads, you manage to translate simple need for clarification into accusations of falsehood or misinformation; sometimes directly, other times by insistently quoting others (e.g. Beaty) who do. If you are truly wondering why these posts generate more "heat" than you expected, you need to examine both how you frame the issues and how you pursue them with others joining the discussion. Your signature line of "Hopelessly pedantic" might be a clue.

    I've set some rather ambitious goals for editing my current textbook before the new year, which means I will not be able to devote substantial time to this thread. Since my sole purpose in participating in these forums is to see how the Lessons In Electric Circuits textbooks may be improved, I will continue my habit of limiting involvement to discussions directly focused on that goal, time permitting. Any attempt to direct this thread into another pedantic disputation will be greeted by silence, whether or not I find the time to respond.

    - Tony
     
  12. Ratch

    New Member

    Mar 20, 2007
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    tony_kuphaldt,

    Good. I think everyone went away from that thread knowing that BJT's are voltage controlled, and respond to Vbe, but useful design techniques and modeling can be had by using the proportionality between Ib and Ie.

    I think it is more than just disagreement on the conditions for which a law applies. It is a matter of calling a definition a law. Ohm's law specifies the material property of electrical linearity, and it takes into consideration that it might fail for some materials that don't follow its rules. That makes it a misnomer to misquote it as as something else, specifically as the resistance definition. The resistance or impedance definition, on the other hand, is always correct no matter what because it is a definition.

    The "electricity" debate is, of course, a debate of about semantics of a word that is so prevalent with so many meanings that it becomes a filler word whose meaning is determined by its context.

    Yes, you have a point there. I am peeved that so many things are misdefined and are misrepresented. Examples include "space walking" and "voltage is electrical energy". When I announce these descrepancies to others, many are insulted by the notion that what they believed all this time is not the way they really are. They usually blame the messenger instead of looking carefully at the message. As for my tag line, it is just an honest statement of a character flaw.

    I hope you will analyze the dispute on its merit and not how it is presented. Even if you choose not to participate, many others will.

    Ratch
     
  13. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
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    Tony, I concur. The current treatise should be updated to include a more thorough unpinning of the BJT, including the voltage-controlled model. It should not be changed to remove the very important treatise on the current-controlled model. I and many others have argued that both models are equally applicable, and there has been no consensus swimming in around this idea of the current-controlled model being invalid - my guess is that those that refute it have never properly used BJTs else it is something to do with their character. The challenge as always is there to disprove it.

    Although this is straying off-topic, like almost all threads that involve Ratch on a high-horse, you have pretty much nailed this in one. On the "Ohms Law" debate, I will post up what I wrote to Ratch over PM regarding Ohms Law (please note that what I say is based on Georg Ohms very paper that he postulated Ohms Law, not an interpretation of Ohms Law):

    In Georg Ohm's own words - Ohm's Law is a statement of proportionality that is a function of various material and environmental parameters. Something that should be straightforward is bogged down in hours of drivel and semantic debates.

    That all said, this thread is about the BJT, so lets keep off-topic discussion elsewhere.

    Dave
     
  14. Ratch

    New Member

    Mar 20, 2007
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    Dave,


    Indeed that is true, and it should be straightforward. I never said otherwise. That does not differ from Ohm's law as I quoted from the two textbooks cited previously: The resistance of metallic conductor is the same no matter what applied voltage is used to measure it.

    The textbook goes on to say that if the resistance is not linear, then it does not follow Ohm's law. Yet most people want to tie the definition of resistance to a material property. That is wrong, not because of any semantic differences, but because it is a misnomer. Will you come out and say that the professors who wrote those textbooks are wrong? Can you find a good college level physics that says uneqivocally that Ohm's law is V=IE?

    OK, the only reason I mentioned it was because Tony did so first.

    Ratch
     
  15. leonidus

    Member

    Sep 23, 2008
    15
    0
    Hi!
    What I have learnt that "BJT is a current-controlled device whereas FET is a voltage-controlled device."
    Accordingto me, this maybe said on the basis of the eqn.s that govern
    their i/p & o/p quantities:-
    1) For BJT: i(C)=
    βi(B)
    &2)For FET(rather JFET):i(D)=I(DSS)*[1-v(GS)/V(P)]^2.
    Although, both of them obey the basic transistor principle:
    "The voltage b/w 2 terminals controls the current through the 3rd terminal." which implies both of them as "voltage-controlled devices"
    -----------------
    L
    EONIDUS:cool:

     
  16. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    The two statements are different, even if the differences are subtle. Ohms Law is stated in Georg Ohm's own words, not an interpretation of Ohm's words like the textbooks you cite. If you read the above, Ohm's Law is merely a statement of proportionality derived through empirical studies.

    I will say it again, in Georg Ohm's own words, Ohm's Law is a statement of proportionality. With that in mind, take a diode which has a non-linear I-V characteristic across a wide voltage range and hence in your world never meets the conditions of Ohms Law (whatever definition you wish to apply); however the diode is locally ohmic, that is over a very small change in applied voltage the current is (effectively) directly proportional to this voltage - and at a fixed applied voltage, in the absence of other extenuating factors as outlined by Ohm, the steady current definition applies and the diode has a resistance that is defined by R = V/I. It is said to be locally ohmic because the I-V characteristic satisfies Ohms' statement of proportionality.

    Any further mention of this off-topic discussion will be removed. If you want to ramble on, start a new thread.

    Dave
     
  17. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    What about transistors in switching mode, multiple emitter transistors, common base connection, emitter current injection, phototransistors,.........the list goes on.

    All this was discussed and proved in Ratch's old thread.

    The collector current is a function of many different variables (including Vbe), all of which can control the transistor, however inconveniently or conveniently, if all the others are held constant.

    That is why the transistor is such a useful device.

    And that is why we need several models and use the most appropriate one for our applications.

    Happy New Year All.
     
    Last edited: Dec 31, 2008
  18. Dave

    Retired Moderator

    Nov 17, 2003
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    I fully concur studiot. In what I am saying to Tony with regards to the e-book material on the BJT, a broadening of the models discussed may be of an advantage to the reader other than the somewhat practically focused treatise that is currently available.

    And to you too; all the best for 2009.

    Dave
     
  19. steveb

    Senior Member

    Jul 3, 2008
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    I believe this claim is true, although I've never verified it myself. I was taught this in school, and I have an old electronics book from the 70s that shows a plot on a log graph. The plot is linear over 9-10 decades.

    However, here's the rub. That basic Shockley ideal equation does not include any frequency limitations of the transistor. It predicts that you can make a 1 GHz amplifier with a basic general purpose transistor. Models are not reality, they are just useful tools.
     
    Last edited: Dec 31, 2008
  20. Ratch

    New Member

    Mar 20, 2007
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    steveb,

    I think that the Shockley diode equation was for DC only. AC applications will have to to take into consideration capacitance, which is considerable for a BJT, especially the junction barrier capacitance. Of course, small signal models do exist.

    Ratch
     
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