Op-Amp modeling

Discussion in 'Homework Help' started by Bigmac, May 7, 2008.

  1. Bigmac

    Thread Starter New Member

    Apr 19, 2008
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    Hello,

    Does anyone know how i could model this op-amp circuit. Any hints or tips. I am struggling to integrate the outside parts of the circuit, with the internal components of the op-amp. I need to include the input and output resistors in my model.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    By modeling do you mean a computer simulation?

    Op Amps match the input voltages with negative feedback, which is key to understanding how they work.

    R3 and R4 form a voltage divider, so the voltage input on the non-inverting input will be 1/2 * V3.

    R1, R2, and R5 are all part of an adder, the output will be -(V1 + V2), assuming V3 is ground. It probably won't be, so I'll leave you to work the rest of it out.

    AAC eBook has several chapters on these circuits such as this one.
    http://www.allaboutcircuits.com/vol_3/chpt_8/8.html
    http://www.allaboutcircuits.com/vol_3/chpt_9/7.html

    One last hint, with a little thought you can apply all of what I've told you, what is in the book, and come up with a simple algebraic equation to describe this circuit. Don't forget to include the resistances in the gain formula.
     
    Last edited: May 7, 2008
  3. Digi Dave

    Member

    Apr 25, 2008
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  4. Bigmac

    Thread Starter New Member

    Apr 19, 2008
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    Thank you for getting back so fast.

    I understand the method you put up, and i am able to get the correct answer. But if you look at the new attachment, you will see what im struggling with. I need to replace the op-amp with the circuit shown. I cant figure out where to put all the inputs (from previous attachment), and how to make them relate to one single dependant source. Im also struggling with the input, and output resistances. Input is very high, and output is very low, but my data sheet gives me a range for each. This will make only a small change in the overall output, but i have to get as accurate as i can. I can only see a small relation to the previous method.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    At this point you lost me. Are you trying to come up with the offset error of the op amp? If memory serves the 741 has a gain somewhere around 10^6 for DC, so the offset between the inputs should be somewhere aound 10e-6 * the voltage.

    Just curious, have you read the AAC eBook section on op amps?
    http://www.allaboutcircuits.com/vol_3/chpt_8/2.html

    It has a model that looks a lot like yours, but with some differences.
     
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