1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

How does a transistor amplify current or voltage?

Discussion in 'General Electronics Chat' started by jac4b, Jul 23, 2010.

  1. Sceadwian

    Sceadwian Banned

    Joined:
    Oct 27, 2006
    Messages:
    14,047
    Likes:
    141
    Location:
    Rochester, US
    During a quantum moment in time current can NOT flow. Zero time as you said. in those moments the currents you're referring to are vectors, which are in fact the voltage fields. Time is not always flowing so current can't always flow, even time has a quanta where things become fuzzy =)
     
  2. BrownOut

    BrownOut Banned

    Joined:
    Jun 10, 2009
    Messages:
    4,630
    Likes:
    60
    Location:
    Huntsville, Alabama USA
    That's incorrect. Current is always flowing. Making a trivial boundary of zero time does not change current. Current isn't a vector, but it does have velocity and direction, even in instantaneous time. A "voltage field" does not arise from current vectors. In fact, there is no such thing as a voltage field.

    Really, this is all a flight of fantasy, and not grounded in any scientific principle. You're only making a trivial condition that doesn’t change anything. Any physicist or engineer will rip this right apart.
     
  3. Sceadwian

    Sceadwian Banned

    Joined:
    Oct 27, 2006
    Messages:
    14,047
    Likes:
    141
    Location:
    Rochester, US
    Poor chose of words BrownOut, my pardon. Electric field.
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. BrownOut

    BrownOut Banned

    Joined:
    Jun 10, 2009
    Messages:
    4,630
    Likes:
    60
    Location:
    Huntsville, Alabama USA

    Sceadwian, I understand the point you're trying to make. You've seen pictures of P-N junctions with all its E fields, energy bands and Fermi levels. It appears when looking at a picture that current isn't flowing and only E fields exist. The reason for that is because there are no "flip" books that explain device physics. Too bad, because then people would get a much better idea of how it actually works. If current actually stopped flowing, the E fields would quickly vanish and nothing would happen. Thus, current is essential and not just some side effect. The other thing you must consider is, even though you see voltages connected to diodes in the books, the V-I relationships are reciprocal, and neither dominates the other. The reason that I and others promote the CC model is, aside from the fact that nobody has yet to factually invalidate it, is because it just works better in BJT's. That in spite of the fact that the voltage model is most often shown in pictures. To really understand BJT's one must get past the pretty pictures and dig deep into the theory ( alternatively, one can just turn voltage and current around and realize they are equally valid and reciprocal results )
     
    Last edited: Jul 27, 2010
  6. Sceadwian

    Sceadwian Banned

    Joined:
    Oct 27, 2006
    Messages:
    14,047
    Likes:
    141
    Location:
    Rochester, US
    That made too much sense not to agree with Brownout.
    I hate being wrong, but I'll eat shoe leather any day rather than muddy the water more beyond this point. I like the viewpoint of reciprocity with voltage/current relationship.


    <edit>After reading this again I realize how horrible a medium the Internet is for sarcasim =\ <end edit>
     
    Last edited: Jul 28, 2010
  7. Ratchit

    Ratchit Well-Known Member

    Joined:
    Mar 12, 2008
    Messages:
    1,934
    Likes:
    78
    Claude,

    Is the the diode equation at the quantum level? It uses voltage to define current.

    How does that apply to the inner control of a BJT?

    Ib and Vbe change simultaneously. The important thing is that a voltage get established across the base-emitter.

    Yes, and in my analogy, the rack and pinion movement is the result of the driver turning the wheel. As I said before, those charges need voltage to move.

    Ic will be able to sustain its changed value unless the depletion width changes. I still believe that Ib and Ic are consequences of Vbe.

    Yes, it takes time to sweep the charges out of the semiconductor when the voltage reverses. That is why I did not want to get involved with diffusion capacitance and its effect on frequency.

    No doubt it is.

    I am sure that different explanations have to be made for high speeds due to the diffusion capacitance. But would you not agree that in the active region, a higher Vbe will increase the Ic or vice versa, regardless of the reason why?

    I sure would have liked to be present during some of those arguments. Especially with the professor.

    Or a minority cult. Do you think the prof is a fool?

    As I said before, they are concerned with design results. VC is not a good design practice in a BJT.

    Ratch
     
  8. Ratchit

    Ratchit Well-Known Member

    Joined:
    Mar 12, 2008
    Messages:
    1,934
    Likes:
    78
    Brownout,

    I stand corrected on the vocabulary, but it does not change anything. Information is in the eye of the beholder.

    It is still not too late to do so.

    You are in the majority, all right.

    The discussion was not about the many ways or the best way to control a BJT. It was about whether a BJT was a CC or CV.

    Ratch
     
  9. BrownOut

    BrownOut Banned

    Joined:
    Jun 10, 2009
    Messages:
    4,630
    Likes:
    60
    Location:
    Huntsville, Alabama USA
    I said nothing about the many or best ways to control a BJT. I detailed why BJT's are CC devices. Look, read and understnad, and you just might learn something.
     
    Last edited: Jul 27, 2010
  10. Ratchit

    Ratchit Well-Known Member

    Joined:
    Mar 12, 2008
    Messages:
    1,934
    Likes:
    78
    Claude,

    I agree with all you said above. That is basic Transistors 101.

    You appear to be a skilled and sure engineer.

    As I said before, I sure would not use Vbe to design something.

    EE is an honorable profession. I don't know why anyone would denigrate it.

    Ratch
     
  11. Claude Abraham

    Claude Abraham Member

    Joined:
    Mar 17, 2006
    Messages:
    197
    Likes:
    23
    Location:
    NE Ohio, USA
    When you say that the diode equation uses voltage to define current, are you referring to Id = Is*exp((Vd/Vt)-1)? Are you aware that the same equation can be written as Vd = Vt*ln((Id/Is)+1)? Just as current is an exponential function of voltage, voltage is also a logarithmic function of current. The equation is usually introduced in I vs. V form to emphasize how a small change in voltage accompanies a large change in current. There is no pecking order here. In Ohm's law, V = I*R, I = V?R< & R = V/I, all say the same thing.

    Since Ib & Ic change before Vbe, how can Ib/Ic/Ie be consequences of the change in Vbe. The consequence cannot occur before the controlling quantity. As far as a higher Vbe increasing Ic, again, the external stimulus increases Ie & Ib first, then Ic & Vbe increase as consequences afterward. Vbe is not controlling Ic, rather Ie is controlling Ic. The sequence of events affirm this. The notion that Vbe controls Ic is one you cannot demonstrate w/ semicon physics, you just dogmatically insist upon it. I'm at a loss to make that clearer.

    Which prof are you referring to? The debates in my undergrad years were not with any prof. An undergrad ME student once told me that V comes before I. He also told me that electric cars should use generators to recycle energy. When the battery is driving the motors, he felt that adding a generator would replace some of the energy, allowing for extended battery life. Attempts to show him why that won't work were futile. My profs always emphasized that I & V are circular in relation to each other. Neither controls the other.

    Do you develop hardware? What field if so? Have you studies semicon physics? You cannot address the energy issues. You ignore the fact that a change in energy requires both Ib & Vbe. It's pointless to continue. People reading this thread will hopefully know that OEMs have a reason for their positiions. They can research this matter themselves, & learn why the CC & QC models are employed for the bjt.

    Claude
     
    • Like Like x 1
  12. BrownOut

    BrownOut Banned

    Joined:
    Jun 10, 2009
    Messages:
    4,630
    Likes:
    60
    Location:
    Huntsville, Alabama USA
    These are the details behind my point about reciprocity.
     
  13. Ratchit

    Ratchit Well-Known Member

    Joined:
    Mar 12, 2008
    Messages:
    1,934
    Likes:
    78
    Brownout,

    You said " And for many, very good, well thought out reasons already given, BJT's, though can be controlled by voltage or current, are considered current controlled devices."

    I took that to mean you were talking about ways to control a BJT.

    Ratch
     
  14. BrownOut

    BrownOut Banned

    Joined:
    Jun 10, 2009
    Messages:
    4,630
    Likes:
    60
    Location:
    Huntsville, Alabama USA
    Then you don't understand simple english. "Are considered current controlled devices" doesn't sound anything like "ways to control BJT's"

    The only mention of control was a subordinate clause. Look it up in a freshman English book.
     
    Last edited: Jul 27, 2010
  15. Ratchit

    Ratchit Well-Known Member

    Joined:
    Mar 12, 2008
    Messages:
    1,934
    Likes:
    78
    Claude,

    Yes, I am aware of inverse relationships. My point is that Vbe and Ic are tied together by a relatively simple equation that does not involve QM to express. I iterate that I would not design a BJT using that equation, however.

    I don't agree that Ib & Ic change before Vce. I would think they changed together. You are right, I cannot imagine one of those variables changing before the other.

    The one I referred to earlier. Here is his website. Department of Electrical, Computer, and Energy Engineering (ECEE) | University of Colorado at Boulder . He responded to me with a email when I asked him if a BJT was CC.

    Ratch,
    I hate to be the one telling you this but the BJT is indeed a voltage
    controlled device. The voltage applied to the base emitter junction controls
    the collector current and the base current is a result of the additional
    hole injection (for an npn BJT) into the emitter as well as the
    recombination in the base-emitter depletion region and the quasi-neutral
    base region. It is tempting to claim that the BJT is controlled by the base
    current, since that is how a BJT is typically biased; the exponential
    variation of the current with the base-emitter voltage makes a voltage bias
    impractical. Any circuit designer will also tell you that any voltage bias
    can be replaced by its Thevenin equivalent current source. Hopefully this
    provides you some ammunition to claim that either one can be claimed when
    treating the device as a black box. Finally, you'll find that a MOSFET
    biased in the subthreshold region has characteristics that are very similar
    to that of a BJT.
    Bart Van Zeghbroeck
    Professor
    University of Colorado
    Department of Electrical and Computer Engineering
    Campus Box 425
    Boulder, CO 80309-0425
    Office ECEE1B41
    Tel: 303-492-2809
    Fax: 303-492-2758
    Email: b...@colorado.edu

    The ME you referred to was off base, of course.

    No, I do not.

    Yes.

    I agree it does. And if the BJT has a higher β, then it requires less energy, less current, but the same Vbe.

    Certainly they can.

    Ratch
     
  16. Ratchit

    Ratchit Well-Known Member

    Joined:
    Mar 12, 2008
    Messages:
    1,934
    Likes:
    78
    Brownout,

    Sorry for the misinterpretation.

    Ratch
     
  17. Mikebits

    Mikebits Well-Known Member

    Joined:
    May 24, 2008
    Messages:
    6,214
    Likes:
    174
    Location:
    San Diego, Ca
    Okay, I tried to bite my lip through this thread but, somebody has to speak up.
    Brownout, surely there are other ways to settle an argument without resorting to ever so slight insulting innuendos. Despite your constant demeaning tone towards Rachit, he has not responded to you in the same manner, and he has reciprocated in a polite and cordial tone. He has not questioned your engineering skills, nor has he questioned your grasp of the English language. Agree with him or not, I think he deserves the same respect that he has shown to you. So how about it?
     
  18. BrownOut

    BrownOut Banned

    Joined:
    Jun 10, 2009
    Messages:
    4,630
    Likes:
    60
    Location:
    Huntsville, Alabama USA
    Oh really? Before you ride in on your high horse again, consider the experienced people who took the time to try to help him understand, and the way he repeatedly threw it back in their faces. Do you think those on here who go to the trouble to help someone should have to put up with impetulence?



    This goes on for another 10 pages or so. Maybe you're OK when our senior people who, overwhelmingly provide the most assistance on here, getting kicked in the teeth for their efforts. I'm not.
     
    Last edited: Jul 28, 2010
    • Like Like x 1
  19. Claude Abraham

    Claude Abraham Member

    Joined:
    Mar 17, 2006
    Messages:
    197
    Likes:
    23
    Location:
    NE Ohio, USA
    Ib/Ie change before Vbe. Look it up. You mentioned diffusion capacitance already. In a cap, the current change leads the change in voltage. Not disputable.

    Now regarding the energy required to change the E fiels, I've shown it is proportional to Ib*Vbe. You replied that for a higher beta, less Ib is needed, but the same Vbe. This is WRONG! The Ib I'm referring to is displacement cuurent. The beta you refer to is conduction current. Not the same. Let's say we have 2 identical bjt devices, equal in all respects except 1, part A, has a base doping density lower than the other, part B. Part A will have a higher beta than part B.

    The energy required to change the b-e region E field is the SAME! The areas are equal,as well as the geometry. Although part A requires less base conduction current than part B, it still needs the same value of displacement current as part B. Which semicon phy courses have you taken? What univ? Which text(s). Don't take it personally, I just want to know your sources for your info. Are you intimately familiar with the difference between conduction vs. displacement current?

    A bjt with very high beta is still a CC device. Take for example, the supergain bjt devices used in the front end of op amps. They have beta values in the 4000 to 5000 range. They are still CC devices. They need current to operate (as well as voltage), but less current than an ordinary bjt with a beta value of 100-400. Likewise, some FETs require more gate drive current than others. A small signal FET operating at 10 Hz, may require only NANOamps of gate drive. A large power FET switching at 2 MHz requires AMPS of gate drive current. It's still a VC device.

    Also, just as bjt parts have varying beta, so do FET parts have varying gm values. A FET may have a gm of 5 mS for given current, temp, die size, etc. Another FET may have 10 times that value, or 100 times. As gm increases, less voltage at the g-s terminals is needed. But as gm increases, the FET does not act like a CC device. It is still VC, but less "V" is needed. This business about "as beta increases a bjt approaches VC" does not hold water. A forward biased p-n junction cannot have zero current with non-zero voltage. A power bjt operating at very high current levels, at low temp, may have a beta value of 3 or 4. With beta = 4, 50A of Ic requires 12.5A of Ib. A supergain bjt at the op amp input can have beta of 5000, so 100 uA of Ic can be had with just 20 nA of Ib.

    They are both CC devices. Likewise 2 FETs w/ grossly differing gm values & gate currents & g-s voltage swings are both VC.
     
    Last edited: Jul 28, 2010
  20. Mikebits

    Mikebits Well-Known Member

    Joined:
    May 24, 2008
    Messages:
    6,214
    Likes:
    174
    Location:
    San Diego, Ca
    Okay, I think your right, I was having a preachy moment.
     
  21. Ratchit

    Ratchit Well-Known Member

    Joined:
    Mar 12, 2008
    Messages:
    1,934
    Likes:
    78
    Claude,

    Sorry for the delay in answering, but it was a busy day.

    In sinusoidal operation, yes. But as I said before, I wanted to just consider the DC characteristics.

    As I said before, I am only considering DC characteristics.

    I took the regular undergraduate courses that are standard for just about any student of EE. I went to a big 10 land grant college. The texts were the modular series on solid state physics from Purdue University by Robert F. Pierret. and Gerold W. Neudeck. It has been a long time since I studied semiconductors intensively, so I am somewhat rusty on the details. I never worked with semiconductors for a living.

    I already granted that current and power is inevitable. My focus is where the control circuit the control lies. It is the driver, or the rack and pinion steering.

    Let's just stick to BJT'S. I already said that power and current are necessary to drive a BJT. Again, my focus is where the control is.

    Ratch
     

Share This Page