OPTs conduction

Discussion in 'Homework Help' started by mellowmon, Mar 8, 2009.

  1. mellowmon

    Thread Starter Member

    Jan 24, 2009
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    Does power output transistor conduct if applied with 150mV at the Base terminal of it with the supply voltage of +/- 130 volts?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Way too much missing information, like the circuit the transistor is in. But, based solely on your information - no. Even a germanium transistor needs more voltage than that to become biased into conduction, again assuming you mean 150 mv on the base relative to the emitter.
     
  3. mellowmon

    Thread Starter Member

    Jan 24, 2009
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    so, now inorder to conduct the transistor there should be a .6 volts across the base and emitter? am i right?

    what is the difference between the quiescent current from Base emitter voltage of .6 volts?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    For conduction to occur, some voltage must be present. The precise voltage will difffer device to device, so the significant condition is insuring that the B-E junction current is limited to a safe value.

    Quiescent current is due to the nature of the circuit the transistor is in, and has to do with biasing. The B-E voltage only has to do with forward biasing. Current in the base is controlled by external devices.
     
  5. mellowmon

    Thread Starter Member

    Jan 24, 2009
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    if quiescent current increase, the Vbe of OPTs also increase?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Once in conduction, the current in the B-E junction is the control. More current in the base, more current in the collector circuit.

    Terminology check - does OPT stand for OutPut Transistor? The prefered term is BJT for a transistor - Bipolar Junction Transistor. It would be more clear to refer to a BJT un the output stage of an amplifier.
     
  7. mellowmon

    Thread Starter Member

    Jan 24, 2009
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    oh, thank you
     
  8. mellowmon

    Thread Starter Member

    Jan 24, 2009
    17
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    ok say for example the driver transistor has a 2.2k emitter resistor, across the resistor the voltage reading is minimum 150mV. so, the Vbe of BJT is also 150mv. now, how to determine the quiescent current?
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    With Vbe = 150 mv, the transistor is not conducting. 150 mv across the emitter resistor means there is 68.2 microamps current through the resistor. A voltage across a resistor only measures the current.

    This might be easier to figure if you can post up the schematic.
     
  10. mellowmon

    Thread Starter Member

    Jan 24, 2009
    17
    0
    Qa,Qb,Q1 and Q2 Vbe are all 650mV
    DC offset is +/-25 mV measured fro the ouput,
    Vbe od Q3 and Q4 is 150mV
    150 mV is also the measured voltage across 2.2 ohm emitter resistor. while VR2 is set to its minimum position.

    When VR2 is adjusted to the midpoint position the reading is 300mV.
    and the full adjustment reads about 550mV.

    Now, can we say that the transistor is conducting with a quiescent current for as low as 68.2mA?
     
    Last edited: Mar 13, 2009
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