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Darlington Pair's Current Direction

Discussion in 'General Electronics Discussion' started by BlackMelon, Jul 6, 2017.

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  1. BlackMelon

    BlackMelon

    188
    5
    Aug 7, 2012
    Hello,

    I do a simulation on the attached schematic. The input is 12-0V square wave with 50% duty cycle at f=8kHz. I want to know the Q1's collector current waveform, so I connect R3 = 1 ohm, and measure the voltage across it. As you can see, when the input is 12V, V+ - V- = 450mV. This means there is a current from the left pin of R3 to the right pin.

    My question:
    When Vin =12V, does this mean the current of Q1 flow from emitter to collector? If so, why does the NPN transistor carry such a current?

    Thank You
    BlackMelon
     

    Attached Files:

  2. BobK

    BobK

    7,680
    1,685
    Jan 5, 2010
    No, it is from the base to the collector. Your 10Ω base resistor is ridiculously low, and this circuit is not operating the way a darlington normally would, it is operating like 2 diodes. There is plenty of current to saturate Q2, so it's collector will be at about 0.2V. This make a current path of 12V signal -> 10Ω resistor -> base of Q1 -> collector of Q1 -> 1Ω resistor -> collector of Q2.

    Try using a 1MΩ resistor as the base resistor and you will see how a darlington normally operates.

    Bob
     
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