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Transistors common what?

Discussion in 'Electronic Basics' started by Bart, Feb 19, 2006.

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  1. Bart

    Bart Guest

    Hello group,
    Is there a "rule of thumb" to choose which set-up for a transistor........
    common base, common emitter, or common collector? Is one better for
    amplification? another for switching? oscillators?
    Thanks in advance,
    Bart
     
  2. Ralph Mowery

    Ralph Mowery Guest

    Most of the time the common emitter is used. The common collector may be
    found as the next most common but way behind the common emitter. The common
    base is sometimes used in low power (preamp for a receiver) radio frequency
    work.
     
  3. There are a whole bunch of conflicting rules of thumb that emphasize
    different things. There are generalities about input and output
    impedances, generalities about voltage and current gain, and
    generalities about frequency response. You have to have some idea of
    how these 3 categories of requirement are prioritized before you can
    select the best configuration.

    For example.
    Highest input impedance = common collector.
    Highest output impedance = common base.
    Lowest input impedance = common base.
    Lowest output impedance = common collector.
    Highest voltage gain = common emitter or common base.
    Lowest voltage gain = common collector.
    Highest power gain (for low frequencies) = common emitter
    Highest current gain = common collector.
    Lowest current gain = common base.
    Highest frequency capability = common base or common collector.

    When you understand at least a bit of why each of these are true, you
    will be on your way toward selecting a configuration that is
    appropriate for various needs.

    So ask about items on this list that make no sense to you.
     
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