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Pnp bjt

Discussion in 'General Electronics Discussion' started by Akshatha Venkatesh, Dec 26, 2017.

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  1. Akshatha Venkatesh

    Akshatha Venkatesh

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    Jan 14, 2017
    In my application, I've used a digital transistor as shown below , but I'm analysing for a case when the Vc of the pnp is more than the Ve and Vb , that is when current flows from collector to emitter in the pnp. What mode of Operation is this ? And how does it impact my circuit which needs a digital transistor.
     

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  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Vc of the pnp transistor should never exceed the emitter voltage.

    Is the load referenced to ground? You don't show that.
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    This mode of operation is called reverse mode.
    Current gain in reverse mode is drastically reduced.
     
    Akshatha Venkatesh likes this.
  4. Akshatha Venkatesh

    Akshatha Venkatesh

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    Jan 14, 2017
    Why should it never exceed the emitter voltage ? What happens if it does ? Yes the load is referenced to ground , I was just analysing a short condition where the collector voltage of the pnp is more than the emitter voltage .
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If there collector is shorted to ground, how has the collector voltage exceed any other voltage in the circuit?
     
  6. Akshatha Venkatesh

    Akshatha Venkatesh

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    Jan 14, 2017
    Sorry I think I gave you wrong information, I did not understand your question correctly and answered it wrongly. I'm just analysing what happens if the collector voltage is more than the emitter voltage. The collector is not referenced to ground.
     
  7. Akshatha Venkatesh

    Akshatha Venkatesh

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    Jan 14, 2017
    But this is only used as a switch, so the current gain shouldn't matter ,right ?
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    So, the circuit isn't relevant to the question?

    The emitter-base breakdown voltage is low compared to the collector-base breakdown voltage. This means that the transistor will break down at a much lower voltage.

    The base-collector junction is very different to the base-collector junction, so although it can insert charge, it is far less efficient.

    The practical upshot is that while you get transistor action, it is very poor.
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

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    Not right. The collector current is still Ic= Ib*gain. This is valid when used as a switch, too.
     
  10. Audioguru

    Audioguru

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    In this circuit I do not see how the PNP collector voltage can be more positive than the +40V emitter voltage. But if it exceeds about +46V the collector-base junction will be forward biased and the base-emitter junction will be reverse biased and will break down which damages the transistor.
     
  11. Cannonball

    Cannonball

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    May 6, 2017
    The collector voltage can never exceed the emitter voltage in a pnp transistor, neither can the emitter voltage exceed the collector voltage in an npn transistor unless it comes from an external source and I have have never seen that.
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

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    ... which may be the case here if the supposed load is forcing a higher voltage. One scenario is e.g. a motor which operates as a generator.
     
  13. Cannonball

    Cannonball

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    May 6, 2017
    I have never seen this in real life. I will analyze the circuit according to theory as I understand it.

    This is a pnp transistor. In order to use it as a switch, the base must be at least .6 volts less positive than the emitter. The base collector junction forms a pn junction that acts like a diode. The base acts like the cathode and the collector acts like the anode. If the anode ( collector) is higher than the cathode ( base) it pulls the base up higher than the emitter thus switching the transistor off and isolating the emitter from the load.

    That is the way I see it.
     
  14. Audioguru

    Audioguru

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    Yes, when it turns on.

     
  15. Cannonball

    Cannonball

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    If the collector voltage is higher than the base by as little as .7 volts you can forget about the npn having any control over the pnp as long as the collector stays .7 or higher.
     
  16. Audioguru

    Audioguru

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    If the PNP is turned on then its collector cannot go much higher than its emitter, then the base voltage will be lower and will keep the transistor turned on.
    If the transistor is turned off then the collector can go higher than the emitter and base, then if the NPN tries to turn on the PNP it will not turn on.
     
  17. Cannonball

    Cannonball

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    May 6, 2017
    If whatever is holding the collector higher than the emitter it will pull the base higher than the emitter and the transistor will not turn on.
     
  18. Ratch

    Ratch

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    What on Earth is a digital transistor? A transistor is a analog device that can be operated in saturated mode ( switch ON), cutoff mode (switch OFF), or active mode for signal amplification. Depending on the values of R3, R4, and the impedance of the load, the PNP transistor shown can be in either of the previously mentioned modes. The current direction in a PNP transistor is from emitter to collector. If you want the current direction to go from collector to emitter, use a NPN transistor. It makes no sense to operate a bipolar junction transistor (BJT) in a reverse manner.

    Ratch
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It is a transistor with inbuilt resistors as shown in his schematic.

    I'm not exactly sure why they call them "digital transistors", but they save 1 or 2 components when you want to drive a small load from a microcontroller pin.

    ROHM has more information (searched for by typing "what is a digital transistor" into google).

    Should I provide this for you too? <grin>
     
  20. Ratch

    Ratch

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    Mar 10, 2013
    Appears to me that the name they gave that component is a misnomer. It is an interface circuit that makes use of transistors and resistors, but it is not a special transistor that operates on digital principles like the name implies. The link also says that it converts voltage to current. Well, doesn't every BJT do that? Anyway, I am sure that DTs are useful in some applications, but they should be named correctly.

    Ratch
     
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