Help with opamp as current regulator

Discussion in 'Homework Help' started by chindigital1, Sep 1, 2010.

  1. chindigital1

    Thread Starter New Member

    May 2, 2010
    2
    0
    It is said this circuit to regulate current through the load. This opamp is connected in openloop config and confusing me.

    How does it work?






    [​IMG]
     
  2. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Why do you say that it is open loop? It is clearly closed loop control.

    Note that the voltage on the + pin of U1 is acting as a reference voltage. An increase in the opamp output will drive the transistor base and cause the emitter current to increase. This increase will cause the voltage on the - pin of U1 to increase. If the voltage on the - pin exceeds that of the + pin, then the output of the opamp will be driven down. This lowers the transistor current and lowers the - pin voltage. This is negative feedback. Assuming that the feedback is stable, then the current in the collector and emitter legs of the transistor will be precisely controlled because the + and - pins of the opamp will have nearly equal voltage.
     
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  3. chindigital1

    Thread Starter New Member

    May 2, 2010
    2
    0
    I thought it was an openloop because there was no feedback(no direct feedback from the output to the -pin).

    Thank you for ur explanation
     
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