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BJT collector

  1. Jan 20, 2015 #1
    Hi i have have a small question regrading to a NPN BJT transistor.

    the DC current gain (beta) is the ratio between Ic/Ib where Ic and Ib are the collector and base currents respectively.

    Ib is the input current, thats alright , but, i dont get why Ic considered as the output?
    in NPN BJT Ic's direction is into the collector (and the electrons direction is the opposite)

    please help me for understanding this issue
  2. jcsd
  3. Jan 20, 2015 #2


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    because Ic is proportional to Ib ( at least within the linear range of the device)

    there's many good www sites explaining the operations of transistors
    some simple googling with give you hours of fun reading :)

  4. Jan 20, 2015 #3

    jim hardy

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    It's purely wording, not science.
    "Input" and "output" usually refer to direction of signal flow, not current flow.

    Look at your three arrangements:
    Common Emitter: Input goes to base, output taken from collector
    this is the arrangement most often encountered
    has both current gain and voltage gain

    Common Base: Input goes to emitter, output taken from collector
    Has voltage gain but not current gain

    Common Collector, or Emitter Follower: Input goes to base, output taken from emitter
    has current gain but not voltage gain

    And observe that current flows into a NPN collector but out of a PNP collector.

    Any help ?
  5. Jan 22, 2015 #4


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    Staff: Mentor

    The controlling signal is the input, the controlled signal is the output. That's a reasonable generalization.
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