Op Amp Simple Circuit Model

In summary: For example, the input resistance is specified by Ri, the gain by A, and the output resistance by Ro. So you would use those values in your analysis, instead of assuming ideal values.
  • #1
monkeysinspace
2
0

Homework Statement



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?


Homework Equations



Kirchoffs's Law and Ohm's Law


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
Hi monkeysinspace, Welcome to Physics Forums.

monkeysinspace said:

Homework Statement



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?


Homework Equations



Kirchoffs's Law and Ohm's Law


The Attempt at a Solution



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

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
gneill said:
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.

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
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.
 
  • #5


Firstly, I would like to commend you for seeking out additional practice problems and trying to understand the concept of op amps. It shows dedication and a desire to improve your skills as a scientist.

In regards to the attached image and the question at hand, it appears to be a simple circuit model of an op amp in an inverting amplifier configuration. In order to find the potential at the inverting input, we can use Kirchhoff's Law and Ohm's Law.

First, let's label the nodes and currents in the circuit. The inverting input is labeled as node A, the non-inverting input is labeled as node B, and the output is labeled as node C. The current flowing through the inverting input resistor (Ri) is labeled as Ii, and the current flowing through the feedback resistor (Rf) is labeled as If.

Using Kirchhoff's Law, we know that the sum of currents entering a node must equal the sum of currents leaving the node. Therefore, at node A, we have:

Ii + If = 0

Next, using Ohm's Law, we can relate the currents to the resistances and voltages in the circuit. For the inverting input, we have:

Ii = (Vi - A(Vi - Vc))/Ri

And for the feedback resistor, we have:

If = (Vc - RoVc)/Rf

Substituting these equations into our Kirchhoff's Law equation, we get:

(Vi - A(Vi - Vc))/Ri + (Vc - RoVc)/Rf = 0

Solving for Vc, the potential at the inverting input, we get:

Vc = (A(Ri+Rf)/(RiRf+RiRo+RfRo))Vi

Therefore, the potential at the inverting input is dependent on the gain of the op amp (A) and the values of the resistors in the circuit (Ri, Rf, and Ro).

I hope this helps to clarify how to work with the attached image. Remember to always use Kirchhoff's Law and Ohm's Law to analyze circuits and to carefully label your nodes and currents. Keep practicing and you will become more comfortable with op amps and other circuit components. Good luck with your studies!
 

What is an Op Amp Simple Circuit Model?

An Op Amp Simple Circuit Model is a simplified representation of an operational amplifier (op amp) used in electronic circuits. It includes the basic components and connections of an op amp, allowing for easy analysis and understanding of its behavior.

What are the components of an Op Amp Simple Circuit Model?

The main components of an Op Amp Simple Circuit Model are the input terminals (non-inverting and inverting), the power supply connections (positive and negative), and the output terminal.

What is the purpose of an Op Amp Simple Circuit Model?

The purpose of an Op Amp Simple Circuit Model is to provide a simplified representation of an op amp that can be easily analyzed and understood. It allows for quick calculations and predictions of the op amp's behavior in a given circuit.

How is an Op Amp Simple Circuit Model different from a real op amp?

An Op Amp Simple Circuit Model is a simplified representation of a real op amp and does not account for all the complexities and imperfections of a real device. It assumes ideal conditions and does not include factors such as input bias currents, offset voltage, and frequency response.

How can an Op Amp Simple Circuit Model be used in circuit analysis?

An Op Amp Simple Circuit Model can be used to analyze and predict the behavior of an op amp in a circuit. It can be used to calculate the gain, output voltage, and other parameters of the op amp, as well as determine the stability and limitations of the circuit.

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