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How transistors really work

Discussion in 'General Electronics' started by sanjoealex, Jul 20, 2004.

  1. sanjoealex

    sanjoealex Guest

    My doubt is that the current in CB junction of a transisitor in
    saturation doesn't flow as in a forward biased diode even though the
    collector base junction is forward biased in saturation. Why is this
    so......???? I have searched many textbooks and sites and still i
    couldn't find an answer. Can anyone help me????
     
  2. Robert Baer

    Robert Baer Guest

    If *both* E-B junction and C-B junction are forward biased, then by
    definition they both carry current, and the device is not acting like a
    transistor - which means the term "saturation" does not apply.
    Now if only the E-B junction is forward biased, and the collector is
    at the same potential as the emitter, then technically the above is
    correct, but the device *acts* as if it were still operational.
    One can set up an experiment by making a small signal one transistor
    amplifier; bias thebase and AC couple a signal in; use a load resistor
    and a variable bias on the collector.
    Lower the collector voltage while monitoring the collector voltage (AC
    and DC).
     
  3. Zorknob

    Zorknob Guest

    The CB current is very small (i.e. doesn't act like a forward biased
    diode) even in saturation because the base is relatively lightly
    doped. The available majority carriers in the base are consumed by
    the emitter/base current leaving no (or few) majority carriers
    available to support collector/base current.

    The majority carriers in the base are attracted to the emitter rather
    than the collector because, even in saturation, the emitter has a
    higher energy potential than the collector. I. e. for an NPN
    transistor at saturation in a common emitter configuration the emitter
    is at ground, but the collector is only at "virtual" ground, so the
    holes in the base are more strongly attracted to the electrons in the
    emitter than the collector.
     
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