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basic transistor action

Discussion in 'Electronic Basics' started by Sunil, Jul 11, 2007.

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  1. Sunil

    Sunil Guest

    Hi....My name is sunil and im doing my electronics engineering in
    India. I have a minor query in the basics of transistors.

    In a npn common base circuit, it says that the collector current Ic is
    normally much greater than the base current Ib. How is that

    i mean, the base and the collector both form the output circuit
    rite....then how can there exist two different currents at two diff
    parts of the circuits??
  2. As long as the collector is more positive than the base, the
    transistor exhibits current gain or beta, with the collector
    current beta times the base current. The emitter carries
    both the base and collector currents.
    Common base means that the base is part of both input and
    output circuits. The input circuit is the voltage between
    emitter and base, while the output voltage is between
    collector and base. Often, in a common base circuit, the
    base is held at a steady DC voltage, with the emitter
    voltage swinging through a very small range, about one diode
    drop more negative than the base voltage, and the collector
    swinging a much larger voltage, centered at some operating
    point quite a bit more positive than the base voltage.
    The output current does not pass through the base, but the
    output voltage swings with respect to the base voltage. The
    collector current passes through the emitter through
    whatever is impressing the input voltage swing on the
    emitter. In the common base configuration, the input and
    output currents are not separate, as they are with the
    common emitter configuration. But the word "common" refers
    to voltage swings. Both the input and output voltage swings
    are with respect to the base voltage.
  3. Eeyore

    Eeyore Guest

    Your mistake is in thinking the base forms an important part of the output

  4. Sunil,

    Think of it as a water pipe connected to a tap (the emitter)
    with the open end being the collector. Now punch a tiny hole in
    the pipe and the leak you have is the base current.

    This analogy has several holes in it (three actually) but never
    mind :)

    Cheers - Joe
  5. neon


    Oct 21, 2006
    there are two diodes in a transistor controlled by inpurity in the base once a transistor saturates current can flow both way.
  6. Sunil

    Sunil Guest

    thanx a lot
  7. Sunil

    Sunil Guest

    thanx a lot
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