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Transistor question

Discussion in 'Home Cinema Building DIY' started by Darren Blake, May 30, 2004.

  1. Darren Blake

    Darren Blake
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    Quick transistor question:

    Which is the best "side" of a transistor to connect the load?

    For NPN, between the + rail and the collector, or
    between the emitter and the -v rail.

    For PNP, between the +rail and the emitter, or
    bwteen the collector and the -v rail.

    Or does it not matter?
     
  2. Chris Muriel

    Chris Muriel
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    Depends very much on what the transistor is driving - the load impedance.
    Since transistors are commonly configured as either common-emitter or common-collector arrangements (common base is rarer - often used in preamps though) , the optimum way will vary.
    Common collector circuits AKA emitter followers have a lower output impedance and emitter load.
    I assume you are referring to the common emitter configuration - which has the load in the collector leg for both NPN & PNP types ; putting the main load in the emitter leg would produce less gain (negative feedback) and have output impedance implications.
    All the transistor's operating current flows through the emitter.
    You should really get a good transistor primer (or find one on the web) to delve further.
    This should have pictures/diagrams which makes things much easier to explain.

    Chris Muriel, Manchester - normally solving applications problems with my employer's ICs (which contain anything from hundreds to millions of transistors internally).
     
  3. Darren Blake

    Darren Blake
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    I probably should have said a bit more about the circuit. It's a simple transistor switch, currently using an NPN transistor wired in common emitter configuraton. I because the input signal is changing from normally low to normally high I needed to swap the transitor for a PNP one. I didn't want to re-configure the circuit so I was wondering whether swapping from NPN common emitter to PNP common collector would cause any problems.

    Anyway, I did it today and it seems to work fine. :)

    Thanks for the reply!
     
  4. Chris Muriel

    Chris Muriel
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    For a simple switch , that shouldn't normally be a problem.
    There's far more to consider when you have an audio, video or RF signal and are trying to correctly match impedances and maintain bandwidth with low distortion.
    With a switch, you're using the transistor in saturated mode so linearity doesn't come into it.

    Chris Muriel, Manchester.
     

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