transistor

Discussion in 'General Electronics Chat' started by shubham161, Sep 4, 2012.

  1. shubham161

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    whether transistor is voltage controlled voltage source or voltage controlled current source?

    If it is voltage controlled current source, then can i produce desired voltage by using some load resistor and a buffer?
     
  2. bertus

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  3. crutschow

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    That question can lead to about 10 pages of discussion on this forum. :rolleyes:

    If you tell us what you are trying to do, we can better help.
     
  4. ramancini8

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    Jul 18, 2012
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    current controlled current source.
     
  5. crutschow

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    The collector of a grounded-emitter BJT transistor will act as a current-controlled current source with the base as the input current. But the current gain is determined by the beta (Hfe) of the particular transistor you are using, which varies from unit to unit. It is also somewhat temperature dependent.

    Do you need an accurate source?
     
  6. Ron H

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    I'm not gonna bite. I'm NOT!
     
  7. Austin Clark

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    Both models are valid. Physically, it's easier to think of them as voltage-controlled, but in practice it's easier to think of them as current-controlled.
     
  8. atferrari

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    Call everybody on board! Here we go...
     
  9. #12

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    You guys gonna do the whole 10 page discussion based on a vague question by a student in India?

    You have more time than I have!
     
  10. crutschow

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    Noticed I said that it "will act" as a current-controlled current source, not that it is. :rolleyes:
     
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  11. ramancini8

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    I guess that I am being led down the primrose path, but in my book Ic=(beta)Ib. This is universally modeled as a base current in the input circuit and a controlled current source in the output circuit. If it walks like a duck, etc.....it is a duck.

    Back in the tube days when I got my start tubes were voltage controlled devices, and circuit designers hated the new current controlled transistors.
     
  12. Ron H

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    OK, I lied. I am gonna bite.
    It's kinda funny, Ron, that you, an op amp guy, would think that way. Differential amplifiers, the heart of op amps, are basically (IMHO) voltage-controlled devices. The base current is an annoyance that has to be dealt with, or at least accounted for.
    I look at them one way or the other, depending on the application. In switching applications, current control is the only model that makes sense. In some situations (log amps, Gilbert cells, current mirrors, etc.) voltage control makes more sense.
     
  13. #12

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    My sentiments entirely. This is the perfect answer from a pragmatists point of view, and I am 99% pragmatist. This attitiude also accounts for my belief that arguing the finer points isn't getting any electrons flowing.

    You guys have fun. I'm going to go melt some solder.:p
     
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  14. ramancini8

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    Now I understand. I too look at the complete circuit as a voltage or sometimes a current amplifier, but I thought that the question was about a single transistor. Nobody has built a single transistor op amp that I know of.

    Yo'all are correct, why cunfuse the newbies with petty bickering.
     
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  15. #12

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    Love it! A student is trying to nail down an easy definition of a transistor because he lacks the experience to know you have to treat it as it is being used and the PhD's didn't have a flame war about it:p
     
  16. Ron H

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    PhD? Piled higher and deeper?
     
  17. #12

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    Yep. Also known as the Texas House-cleaning method. If it won't fit in the closet, pile it higher and deeper.

    (Expecting Bill Marsden to drop by and say, "Hey! I resemble that remark!")
     
  18. Ron H

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    Yeah, lookin' at that shiner he's sporting, he does resemble something that's been piled higher and deeper.:D

    Please don't ban me, Bill!

    EDIT: I didn't realize that it sounded like I was referring to excrement. I just meant that it looked like HE had been piled higher and deeper.
    Maybe I should just delete this post while I can.:(
     
    Last edited: Sep 5, 2012
  19. #12

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    You're living dangerously, Ron!:rolleyes:
     
  20. cabraham

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    Annoyance? Couldn't the same be said for Vbe? An ideal p-n junction in the forward direction has zero voltage drop for any finite current value. Second, the current controlling Ic is emitter current Ie, not base current Ib.

    Thirdly, base current may not contribute directly to Ic, but it has a useful purpose. Every bjt can be fabricated for "supergain" behavior, beta value can be 4,000 to 5,000 typically. The base current exists due to hole density in the base. When b-e junction is forward biased, holes drift from base to emitter, recombining w/ emitter electrons.

    The value of this hole drift is determined by the density of acceptor atoms which is controlled during the fabrication process. A sparse acceptor density & very thin base region results in great current gain values, beta. But designing the device for superbeta performance involves tradeoffs.

    With thin base dimensions, & sparse acceptor density in the base, Vce blocking capability is low, & collector to base leakage current is high. Leakage from collector to base is very undesirable, it hurts performance while providing no benefit. Also, punch through when Vce breakdown occurs is very undesirable.

    So how we avoid these 2 bad behavior issues, is to increase doping density of acceptor atoms in the base region (npn device), which reduces c-b leakage current, & widen base region, which increases Vce blocking ability. The price paid is lower beta & higher base current, but it is worth it. Again, Ib does not directly contribute to Ic, but it mitigates these 2 problems I discussed.

    I would rather have a bjt that required a little more base current but exhibits less c-b leakage, as well as not exhibit Vce punch-through. I'm sure you don't want those either.

    Likewise, Vbe is an annoyance. In logic, output swing for an emitter follower cannot go to the rail because of Vbe. If Vbe were 0.01 V, life would be better. A power amplifier exhibits crossover distortion because of Vbe value of 0.7V. If it were 0, crossover would have lower distortion.

    Germanium bjt devices have lower Vbe, a desirable feature. But their collector-base leakage current is much greater than silicon, esp at high temp. With Si, we accept the higher Vbe in return for better c-b leakage current values. It's a tradeoff indeed.

    Finally, if a bjt amp stage is configured as an emitter follower, a common use, Ib combines with Ic & Ie is the load current. In an EF, Ib is not going to waste, it drives the load. Many standard npn linear regulators use an EF stage for the output. The low dropout regulators use either a pnp bjt or P MOSFET stage. The configuration is common emitter or common source. With CE, Ib does not drive the load.

    Anyway, I had to clear up this urban myth that Ib is just an annoyance. I studied semiconductor physics including fabrication, & believe me, if we have had the ability to produce bjt parts with beta of 5,000 since the 60's, but we generally opt not to, you know there is a good reason. If higher base current was merely an annoyance, it would have been reduced in every device to the extent feasible.

    OEM producers of bjt intentionally lower beta, & increase base current, because there is something to be gained by doing so. Higher acceptor atom density reduces c-b leakage, & longer base region distance increases Vce blocking value. These are good things - I'm sure you would agree. BR.

    Claude
     
    Last edited: Sep 10, 2012
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