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How a BJT Transistor works (base current version)

A simple (if not absolutely accurate description) of how a normal transistor operates

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  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    (*steve*) submitted a new resource:

    How a BJT Transistor works (base current version) - A simple (if not absolutely accurate description) of how a normal transistor operates

    Read more about this resource...
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    You want to go to war with LvW? ;)

    Mind you: although the Ebers-Moll modell does describe Ie=f(Vbe), I agree that this is a theoretical view not practical in everyday work. I, too, use the "base current driven BJT model" in my daily work.
     
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  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Actually, think LvW and I are in broad agreement. The only argument (as I understand it at the moment) is whether we call it voltage or current control.

    Mind you, I don't want to put words into his mouth.
     
  4. Laplace

    Laplace

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    Why would two separate resources be needed to explain how the BJT works when it works in only one way? If the way we used transistors was to directly apply a specific voltage to the base, then the (Vbe) explanation would be appropriate. However, in all but some very special cases the way we use transistors is to force a current through the base and let the junction generate its own Vbe (there is an equation for this).

    With the (Vbe) explanation you can't just do a lot of hand waving but must quantify each step. First is the relation of Ib to Vbe, then the relation of Vbe to the depletion region, then the calculation of how the depletion width affects the diffusion process which ultimately determines Ic. Then it must be shown why Ic is proportional to Ib.

    With the (Ib) explanation you just start with the observable fact that Ic is proportional to Ib. And the fact that Vbe won't change much over the range of interest. Which is a moot point because the art of biasing a transistor in its operating region strives to eliminate the effects of changes in Vbe or transistor beta values.

    So it is not two different methods of BJT operation, but rather what you choose to leave out of the explanation. Because some facts are just pedantic and useless.
     
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  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Because one way concentrates on a method which is undeniably practical and the other describes it in a way that is arguably more accurate and which can help explain some transistor behaviour that the former cannot,

    Secondly, it means that we won't have to have voltage vs current arguments every time this topic crops up.

    Thirdly, once the above 2 objectives have been met, bringing peace to the Middle East will be a doddle.
     
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  6. Arouse1973

    Arouse1973 Adam

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    Yep I also use the base current method.
     
  7. Arouse1973

    Arouse1973 Adam

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    This is really good Steve. When I first started out in electronics, I used the words Not Pointing iN for the arrow head on a NPN and the other one was a PNP.
    Cheers
    Adam
     
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  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Updated (finally).

    I welcome any criticism.
     
  9. LvW

    LvW

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    May I add one remark (because mentioning LvW in this thread):
    For my opinion, it helps to make clear if "the method" is applied (a) for explaining the function of the BJT or (b) for designing a BJT-based amplifier.
    (By the way: I think, mixing both items during relevant discussions always creates misunderstandings).
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Thanks for your comment. Does this reflect something in the resource I have written, or is it a general observation?
     
  11. LvW

    LvW

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    For my opinion - both (as well as).
    In your contribution you are referring to Horowitz/Hill page 64. But one should consider that on page 62 the authors speak about a "very simple introductory transistor model" only.
    (Comment: A model does not necessarily reflect the main physical properties; it is the only task of a model to work properly).
    More than that, on page 79 we can read: "Clearly, our transistor model is incomplete and needs to be modified" ..."the transconductance model will be accurate enough for the remainder of the book". (As you know, the transconductance model is based on voltage control). Hence, we can conclude that this model is in accordance with physical realities.

    May I add one comment? It is funny, however, everybody who claims to use a "current-control model" during the design steps is in error. That means: He believes to use the current-control feature but, in fact, he doesn`t.
    Let me explain: Everybody designs a BJT stage based on a certain base-emitter voltage to safely open the BJT to allow a DC current Ic. Is this current-control? Of course, not.
    More than that, for calculating the base bias circuitry we, of course, take into account the fact that there is a current Ib (loaded voltage divider principle) - but has this approach anything to do with control?

    Therefore, my question: At which stage of the design process some calculations are based on a current-control feature? More than that, to what extent a current-controlled model would simplify calculations (as claimed by many defenders of this model)?
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    You will note that in the very first section there is place to put a link to an simple explanation anyone might want to write which deals with control of transistors by varying Vbe.

    So apart from this, is there anything specific that you'd change in this description?

    And by the way, I am one of those that *know* that the control of the transistor is by varying Vbe, so you're preaching to the choir.
     
  13. LvW

    LvW

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    OK - I will check.

    OK - but in this case I must ask : Why in the title of your text the phrase "base current version" ?
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Because I am describing the operation in terms of the base current.

    As you and I both know, that has limitations, but it is an easy way to introduce people to the concept.
     
  15. LvW

    LvW

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    Steve, what shall I answer? You know of my opinion: It has no "limitations", but it simply does not reflect the reality and it does not simplify anything (as I have shown in my former answer).
    It is even worse - it directs a beginner to a wrong direction so that he cannot explain and understand, for example, the principle of RE feedback. Isn`t it even a contradiction to require a certain voltage VBE (to allow a quiescent current Ic), and to state - at the same time - that any change in this current is caused by a change in IB ?

    You have asked for some comments to your contribution (besides the discussed control mechanism):
    * I think, such an introductory text should contain the keyword "biasing in the active region" with the aim to fix an operating point in the quasi-linear part of the Ic=f(Vbe) curve.
    (Sorry, but again the role of Vbe is involved automatically).
    * For my opinion, the shown circuit does not help too much because it never will be used in practice (as an amplifier). Why not discussing a more realistic circuit that contains biasing resistors?
    * The voltage of 0.6 V is mentioned without explanation.
    (Again we touch the control mechanism question; now you can verify perhaps why I cannot see any advantage or simplification in the attempt to explain the BJT function using IB as the controlling quantity.)
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That's absolute rubbish.

    It is perfectly compatible with that.

    For example

    With a base resistor Rb, an emitter resistor Re, and a collector resistor Rc like this:

    RbReRc.png

    Assuming

    Ic = Ib × hFE
    Then

    Vin = (Rb × Ib) + VBE + ( (Ib + Ic) × Re)
    = (Rb × Ic ÷ hFE) + VBE + ( ( (hFE + 1) ÷ hFE) × Ic) × Re)
    ≈ (Rb × Ic ÷ hFE) + VBE + (Ic × Re) -- assuming hFE >> (Rc ÷ Re)​
    and

    Vout = Vcc - (Ic × Rc)​

    So (rearranged)

    Ic = (Vcc - Vout) ÷ Rc

    Thus

    Vin ≈ (Rb × ( (Vcc - Vout) ÷ Rc) ÷ hFE) + VBE + ( ((Vcc - Vout) ÷ Rc) × Re)​

    Therefore

    Vout ≈ ( ( (Re × hFE + Rb) × Vcc) - (Rc × hFE × (Vin - VBE) ) ) ÷ (Re × hFE + Rb)​

    If Re = 0, this becomes

    Vout ≈ ( ( Rb × Vcc) - (Rc × hFE × (Vin - VBE) ) ) / Rb
    = Vcc - (Rc × hFE × (Vin - VBE) ) ÷ Rb
    = Vcc - ( (Vin - VBE) ÷ Rb) * hFE * Rc

    And I gave in the resource:

    Vout = Vcc - ( (Vin - VBE) ÷ Rb) × hFE × Rc -- Albeit with slightly different nomenclature​

    If Rb = 0, you get

    Vout ≈ ( ( Re × hFE × Vcc) - (Rc × hFE × (Vin - VBE) ) ) ÷ (Re × hFE)
    = Vcc - (Vin - VBE) × (Rc × hFE) ÷ (Re × hFE)
    = Vcc - (Vin - VBE) × (Rc ÷ Re)​

    And therefore the gain is

    - Rc ÷ Re -- where hFE >> (Rc ÷ Re)​

    Which is, I presume, approximately where you suggested base current would not get me.
     
    Last edited: Oct 13, 2014
  17. LvW

    LvW

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    Steve - really absolutely rubbish? Are you denying that Re implements current-controlled voltage feedback?
    I have several years experience in teaching electronics - what do you think how students would react upon your method to explain the principle of RE feedback? (The emphasize is on "explain" !). With formulas I can do everything - I even can exchange Vin with Vout and claim that Vin is controlled by Vout.
    With other words: Formulas can only describe a relation between quantities without any information about cause and effect. That`s an old wisdom.

    Here comes my explanation how Re feedback works:
    A rising Ic (temperature, tolerances) causes a voltage drop increase across Re - and, thus, reduces VBE which acts back to Ic and reduces its increase.
    As you see, it is one single sentence which can be easily understood. Rubbish?

    Final remark: You have asked for some comments to your text. That´s what I have done. If it is really "rubbish" - forget it!
     
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    And exactly the same sentence could be applied to what I showed. I don't see your point.

    And the formulas don't show what is controlled by what, they show the relationship. -- hey we agree on something.

    Please feel free to write a resource describing the transistor in terms of voltage control.
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I would consider doing that in another resource discussing the use of a transistor as a (practical) amplifier.
     
  20. LvW

    LvW

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    OK - where is your explanation (in words!) for the phenomenon of RE feedback - without using the fact that VBE controls Ic ?
    I don`t intend to write such a paper. Since several decades, such a description can be found in several books and other resources.
     
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