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common emitter/base/collector -- remembering which one is which

Discussion in 'Electronic Basics' started by John Popelish, Apr 9, 2007.

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  1. I think this is a very wrong way to look at the
    configurations. The common node is common to both input and
    output.

    Base is always part of input, collector is always part of
    output, but:

    In a common emitter configuration, the input is applied
    across base to emitter, and the output is taken across
    collector and emitter.

    In the common base configuration, the input is applied
    emitter to base, and the output is taken across base to
    collector.

    In the common collector configuration, the input signal is
    applied across base to collector, and the output is taken
    across collector to emitter.

    The common terminal is just the one that essentially holds a
    constant voltage (an AC common) while the other two swing
    with the input and output signals.
     
  2. vlsidesign

    vlsidesign Guest

    I've heard that the easy way to remember which configuration is being
    used is that the "common" node is basically the node that is not being
    used as the input or output. It seemed to help me but didn't know if
    there are "traps" or "warnings" in using this to remember.
     
  3. John  Larkin

    John Larkin Guest

    That's about right; there's no signal on the "common" node.

    One trap would be something like a phase splitter, where the base is
    the input but both the emitter and collector are outputs. Or some RF
    oscillator circuits where everything is everything.

    I seldom hear people say "common collector." Most say "emitter
    follower", which really should be "base follower."

    But these categories are gross simplifications; the transistor does
    what it does.

    John
     
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