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What happens before and after the saturation of bipolar transistor?

Discussion in 'Electronic Basics' started by nabi, Mar 1, 2007.

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  1. nabi

    nabi Guest

    If base current keeps increasing, then the collector current increases
    upto the saturation point. Books say.
    What happens if the base current does not decrease at the saturation
    The transistor blows up or there will be no collect current flow?

    Secondly, when the saturation happens at the collector side, the
    volate of the collector point becomes zero volt (ground), so it can be
    used to be a temporary ground for another path? A flash light circuit
    - which has two LEDs and two bipolar transistor and capacitors on each
    collector side, runs this way.

    Thanks in advance.
  2. The transistor saturates as the collector voltage swings to
    the emitter side of the base voltage. For example, an NPN
    with grounded emitter and a collector resistor to positive
    supply voltage...

    As the base current rises, the collector current rises many
    times faster, because of the current gain of the transistor.
    The base to emitter junction acts much like any other
    forward biased diode, so that the base voltage is about .6
    volts more positive than the grounded emitter.

    But as the collector current rises, the collector voltage
    falls toward the ground rail, as more and more current is
    passed through the collector resistor. Once the collector
    voltage falls to below the base voltage, the collector to
    base junction is no longer reverse biased, and the
    transistor is said to be in saturation.

    If you keep raising the base current, the collector voltage
    falls so much that the collector to base junction becomes
    forward biased enough that a significant fraction of the
    base current passed directly to the collector, making it
    difficult for the current gain of the transistor to pull the
    collector any lower. In effect, a significant part of the
    collector current load becomes base current that poured
    directly into the collector.

    The swing from base collector junction reverse biased to
    forward biased in the defining change that causes saturated
    operation to be different from unsaturated operation.
  3. Guest

    The transistor will blow up. If you connect your voltage to the base
    without a limiting resistor, poof, your transistor will be blown, you
    should try it :)

    Not sure what is the question?
  4. Rich Grise

    Rich Grise Guest

    This is kind of ambiguous. "AT" the saturation point, increasing the
    base current can't saturate the transistor any more, it will just
    dissipate power in the base, eventually melting it.
    The transistor can be damaged if there's excessive emitter-base flow,
    but other than that, the transistor will just stay in saturation.

    When you reduce the base drive, of course, the collector current
    will decrease - this is called the "linear region".
    Approximately, yes. The transistor's data sheet will show you Vcesat -
    that's the collector to emitter voltage at saturation.
    Yes, if you account for Vcesat. This is usually called a "low-side switch".
    You seem to have answered your own question. :)

    Have Fun!
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