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Bjt as aswitch

  1. Jun 23, 2007 #1


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    i want to know why at some point the low output voltage for a bjt biased between 0 and 5
    on the base
    and voltage 5 at the drain
    ranges between 0.225v and 75mv so which of them is the low voltage pz answer me now i have a project to deliver
  2. jcsd
  3. Jun 23, 2007 #2


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    A BJT has three terminals, EMITTER, BASE, COLLECTOR,
    There are two varieties of BJT:

    NPN and PNP which represent differeing doping polarity used in making the devices sections.

    A BJT transistor driven into fully saturated conduction
    will usually have a relatively small voltage drop between
    its COLLECTOR and EMITTER; the value of that voltage
    is called "V_CE_SAT" for "saturated Voltage between
    COLLECTOR and EMITTER" and it is often around
    0.35 Volts.

    When there is little BASE current the transistor will present
    a very low conductance (i.e. a high resistance) between
    COLLECTOR and EMITTER, and thus the voltage drop
    across the COLLECTOR and EMITTER will be large, almost
    as large as if the transistor was taken out of the circuit
    and a large -- e.g. 50 million ohm -- resistor is placed
    between where the collector and emitter terminals were.
  4. Jun 23, 2007 #3


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    And usually one doesn't speak of "low" voltage with
    a single BJT since usually a logic circuit involves
    at least one BJT and one diode and at least some
    resistors or other BJT components.

    However comparing magnitudes, 0.075V is LOWER than
    0.225V; I suspect 0.225 could be V_CE_SAT, I have
    no idea what 0.075V represents....
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