Op-Amp output calculation

Discussion in 'Homework Help' started by StuckStudent123, Sep 23, 2016.

  1. StuckStudent123

    Thread Starter New Member

    Sep 23, 2016
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    Hi,
    Is anyone able to help me with this op amp calculation?
    It's the 1k resistor that's confusing me a bit. If it wasn't there I'd imagine it would just be a simple differential amplifier (but I might be wrong there too:D).
    Rx=2k, V1=3v, V2=9v and the question asks for V0.
    Thanks
    [​IMG]
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    I can't open the picture.
     
  3. StuckStudent123

    Thread Starter New Member

    Sep 23, 2016
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  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    That one requires me to log in.
     
  5. StuckStudent123

    Thread Starter New Member

    Sep 23, 2016
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    Third time lucky!
     
  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    OK!
    What is the voltage at the inverting input? Hint: No calculation required.
    Given that, you know the currents in Rx and the 1k resistors and therefore the current in the 2k resistor. Now you can calculate Vo.
     
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  7. StuckStudent123

    Thread Starter New Member

    Sep 23, 2016
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    So do you not have to do anything with V1, the non-inverting?
     
  8. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    You most certainly do!
    The output voltage is the difference between the input voltages multiplied by the gain of the amplifier. If we are taking the gain of the amp as infinite, what does this tell you about the voltage difference between the inputs?
     
  9. StuckStudent123

    Thread Starter New Member

    Sep 23, 2016
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    That it's almost 0?
     
  10. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Yep, and you know the voltage at the non-inverting input so...
     
  11. StuckStudent123

    Thread Starter New Member

    Sep 23, 2016
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    They're the same?
     
  12. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Yep.
    Now you know the voltage at the non-inverting input, you can calculate the current through Rx and R1
    And what is the current into the non-inverting input of the amp?
    And therefore what is the current in R2?
     
  13. #12

    Expert

    Nov 30, 2010
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    Any op-amp running properly in a closed loop gain scenario will null its inputs.
     
  14. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Important result to remember.
     
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