# Homework Help: Op Amp Simple Circuit Model

1. Mar 25, 2014

### monkeysinspace

1. The problem statement, all variables and given/known data

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

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2. Mar 25, 2014

### 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.

3. Mar 25, 2014

### monkeysinspace

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.

4. Mar 25, 2014

### 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.