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Why does connecting a bjt backwards work?

  1. Mar 27, 2012 #1
    I'm trying to understand transistors so I bought a couple and have been doing some reading, I basicly got it but some quick experiments have me dumbfounded..

    I have the most basic NPN transistor circuit setup http://amasci.com/amateur/transis2.html like thatone, only with a loudspeaker instead.. Now, why does it still work as expected if I simply flip/reverse the transistor so the collector connects to the '-'poles on the batteries?

    This is actually the behaviour I've expected since NPN is symetrical, But I've understood that there is in fact a difference between the collector and the emitter. Enlighten me!!
  2. jcsd
  3. Mar 27, 2012 #2


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    You are right that most bipolar junction transistors will "work" when the emiitter and collector are swapped over. As you said, conceptually the 3-layered structure "looks the same" from either side, even though the physical structure does not have symmetrical geometry, and the level of doping (deliberate impurities) in the two N regions is different.

    The transistor will only match the parameters in its data sheet if it is connected the right way round. Otherwise, you will probably get lower gain (beta), higher electrical noise and leakage current, lower power dissipation limit, etc.
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