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transistor question

Discussion in 'Electronic Basics' started by Scott, Apr 15, 2004.

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  1. Scott

    Scott Guest

  2. Here is a picture of the symbols with the lead identity.
    http://www.americanmicrosemi.com/tutorials/bipolartransistor.htm
     
  3. here are the rules for bipolar transistors:

    1) The leg with the arrow is the emitter.
    2) The leg that goes perpendicular to the line is the base
    3) The other leg is the collector (the one with no arrow)
    4) If the arrow points into the line, its PNP, otherwise, its NPN.

    This arrow thing is also used in the diagrams for MOSFET and JFET devices.
    If the arrow on the gate is pointing towards the line, its an N-CHANNEL
    device. Otherwise, its a P-CHANNEL device.

    Here is the reason why this is the case:

    Bipolar transistors are really diodes with an extra hunk of silicon added.
    The symbol for a diode is an arrow going from positive to negative (the P-N
    junction)

    These arrows in transistors indicate where and which way the diode is
    facing. So, for an NPN transistor, the arrow is pointing out of the
    transistor. For these transistors, the arrow points from the base (which is
    P type silicon) to the emitter (which is N type silicon). A diode is a PN
    junction, so its really just the diode which is depicted by the little
    arrow. The collector is the line which doesn't have an arrow. Its not really
    used as a diode in the transistor; its 'reverse biased', and only when you
    put current through the actual diode (the base to collector) does current
    flow through the collector.

    For a PNP transistor, its really the same, except that the diode is now from
    the emitter (which is P type silicon) to the base (which is N type silicon).
    A diode is always a P to N junction. Thus, the arrow points from the emitter
    to the base, into the transistor. Again, the collector is not a diode in the
    traditional sense, and thus doesn't have an arrow. Again, the collector is
    'reverse biased', an NP junction, which requires current flowing through the
    actual diode (the emitter-base junction) to cause current to flow across it.

    Hope this lessens some confusion.

    Regards,
    Bob Monsen
     
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