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transistor wired like this

Discussion in 'General Electronics Discussion' started by spencer_manzon, Jan 8, 2011.

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  1. spencer_manzon

    spencer_manzon

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    Jan 8, 2011
    trnpn.GIF

    if I wire a transistor like this, what would happen? :)
     
  2. barathbushan

    barathbushan

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    Sep 26, 2009
    Well when you apply base voltage , both of them will turn on simultaneously, and then conduct in a parallel fashion
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    True, but I wouldn't plan on using it to switch more than one of the transistors can comfortably cope with.

    Thermal runaway will eventually cause one transistor to be passing the bulk of the current.

    If you want to do this, the usual approach is to place resistors in series with the emitter such that they drop about 0.1 to 0.2 volts at max load.
     
  4. barathbushan

    barathbushan

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    Sep 26, 2009
    I think this is a theoretical question , from a text book . And in theory anything can happen !!
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    In theory, only one transistor would turn on due to differences in Vbe, but practically, both will turn on, but by different amounts. The one that's turned on more will carry more current and get a further lowered Vbe, robbing yet more current from the other transistor, etc, etc.

    You can do this more easily with mosfets (as long as you provide a gate voltage that well exceeds either's Vgs thresholds) but it is fraught with problems if you do it with bipolar transistors.

    And that's both in theory AND in practice.
     
  6. spencer_manzon

    spencer_manzon

    3
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    Jan 8, 2011
    wow! thank you for the replies... this really comes from a textbook? i wired transistors like that hoping it would amplify better than with a single one... i know about darlington pairs but I wanted something new..

    valuable facts are presented here, thank you :)
     
  7. barathbushan

    barathbushan

    223
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    Sep 26, 2009
    yes i have seen it in many textbooks and datasheets , but like steve mentioned above , connecting them in parallel would lead to one transistor conducting heavily , and the other transistor will do very less work .

    I think you are mistaken here , parallel connection is made only to increase the current output of the transistors .

    Also if the above circuit is ever to be constructed . just place a 1 ohm resistor at the collector of both the transistors , and place diodes at the collectors such that current only flows out or sources from the collector , doing this will ensure safety of the transistor
     
  8. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    That sounds like a rather untraditional & inefficient method, one I have never seen recommended or done anywhere else in my life.
    You can't refer to a specific ohmic value w/o knowing the current that'll flow.
    The diodes won't protect against imbalance or anything but reverse voltage; where the load generates more voltage than the PSU (and that does not include inductive kickback).
    The recommended way (not without reason) is like steve says emitter resistors, dropping a few tenths Volts at the nominal current flow.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    And to amplify on Resqueline's point, The reason the resistor is in the emitter and not the collector is that reducing Vbe will cause a transistor to turn off. The resistor in series with the emitter provides negative feedback, tending to turn the transistor off a little as the current increases. This tends to balance the current. This behaviour is far more important that the increased voltage drop seen by the load (which would be the effect of placing it in the collector).

    And I can't see any point in placing diodes there...
     
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