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Homework Help: Op Amp Simple Circuit Model

  1. Mar 25, 2014 #1
    1. The problem statement, all variables and given/known data

    Please check attached image.

    I am doing some op amp practice problems and I came across this problem:

    I have to find the potential at the inverting input of the simple circuit model of the op amp. The op amp would be a basic inverting amplifier. I have to give the answers using A, Vi, Ri, Rf, Rd, Ro. There are no loads.

    I am familiar with how to work with an inverting amplifier similar to here: http://www.electronics-tutorials.ws/opamp/opamp_2.html

    However, how do I work with the attached image?

    2. Relevant equations

    Kirchoffs's Law and Ohm's Law

    3. The attempt at a solution

    Have it done by hand, but pretty much this: http://www.electronics-tutorials.ws/opamp/opamp_2.html

    Attached Files:

  2. jcsd
  3. Mar 25, 2014 #2


    User Avatar

    Staff: Mentor

    Hi monkeysinspace, Welcome to Physics Forums.

    The circuit in your attached diagram is the Equivalent Circuit Model for your op-amp. Imagine that it fits inside the op-amp triangle of the inverting amplifier circuit. Note that the model does not assume that the op-amp is ideal! There's finite input resistance, finite gain, and non-zero output resistance in the model.

    Replace the op-amp triangle with this equivalent circuit and analyze.
  4. Mar 25, 2014 #3
    How would I go about showing the finite input resistance, finite gain and non-zero output resistance? Would I let them equal ideal values?

    In terms of what I have now, It's something along the lines of

    Vout = -(Rf/Ri)Vinput

    Not exactly sure where the additional parameters from the equivalent model fits in. I'm fairly new to op amps, so this is my first dabble in it. Perhaps that's why it's not clicking.
  5. Mar 25, 2014 #4


    User Avatar

    Staff: Mentor

    You want to replace the op-amp symbol in your circuit with the equivalent model, then analyze the resulting circuit for the required quantities. Note that the equivalent model contains components that specify the particular parameters.
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