How Do You Calculate the Equivalent Resistance in an Op Amp Circuit?

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In summary, the student is trying to find the R equivalent of a circuit and is having trouble understanding the concepts. The student changed the first resistor to see if that would help and it did. The student then used the voltage difference between the + and - inputs to find the current.
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Innuendo
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[SOLVED] Op Amp Requivalent Please help!

Homework Statement


I'm working on Ideal op amps and I need to find the R equivalent of the system.
I'm not sure if this term is used in other places but R equivalent = total Resistance of the circuit.

http://img169.imageshack.us/img169/4386/reqrk6.png

Homework Equations


The Attempt at a Solution


I have to place a voltage at the circle at the top left and use the voltage to divide the current to get the Req, but I don't know how to find the current. I think I have to use a current divider at the 10 and 40 k ohms but I can't seem to get the answer of 14 k ohms as the answer in the back of the book. Please help!

I'm also confused if this is the Negative Resistance mentioned in the book. But the given equation for Req = -(R1/R2)R
 
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  • #2
You say "I think I have to use a current divider at the 10 and 40 k ohms...", but I don't see a 10k resistor.

However, I changed the very first resistor to the right of the little circle to 10k, and I got 14k for Req. Here's how I did it:

Apply 1 volt to the little circle. The bottom branch of 40k and 30k form a voltage divider. The voltage at the + input of the opamp will be 3/7 volt, and if the opamp has infinite gain, the voltage at the - input will also be 3/7 volt. This means the voltage across the 40k resistor in the bottom branch will be 4/7 volt, and the voltage across the 10k in the top branch will also be 4/7 volt. The current in the 40k in the bottom branch will be (4/7)/40000 and the current in the 10k in the top branch will be (4/7)/10000. The total current drawn from the applied 1 volt will be (4/7)/40000 + (4/7)/10000 = 7.142857E-5 amps. Then Req = 1 volt / 7.142857E-5 = 14000 ohms.
 
  • #3
ohh I see! I think you're right, I misread it when I drew that on paint, the top resistor is 10k ohms. But one thing I didn't quite get was why the voltage across the 10k resistor is also 4/7. Thanks for helping.
 
  • #4
When you're solving opamp problems, unless the problem says otherwise, they almost always assume the opamp is ideal: it has infinite gain and the inputs don't draw any current (or negligible current at most). This means that if the opamp is operating normally and the output isn't driven to the power supply rails (which is usually the case if the negative feedback is greater than the positive feedback), then the + and - inputs will be at the same voltage (will have no voltage difference between them). These two assumptions will allow you to easily solve a lot of opamp problems.
 
  • #5
Thank you.
 

FAQ: How Do You Calculate the Equivalent Resistance in an Op Amp Circuit?

What is an op amp equivalent circuit?

An op amp equivalent circuit is a simplified model of an operational amplifier (op amp) that is used for analysis and design purposes. It is composed of ideal components, such as a voltage source and resistors, that represent the behavior of a real op amp.

Why is an op amp equivalent circuit useful?

An op amp equivalent circuit is useful because it allows for easier analysis and design of op amp circuits. By using ideal components, calculations become simpler and the behavior of the op amp can be better understood.

How is an op amp equivalent circuit different from a real op amp?

An op amp equivalent circuit is different from a real op amp in that it is composed of ideal components, whereas a real op amp has non-ideal characteristics such as input offset voltage, input bias current, and finite gain. These non-idealities are not present in an op amp equivalent circuit, making it easier to analyze and design with.

Can an op amp equivalent circuit be used for all op amp circuits?

No, an op amp equivalent circuit is meant to be used for simple op amp circuits. It does not take into account more complex op amp circuits, such as those with feedback loops or multiple stages. In these cases, a more detailed model of the op amp is needed.

How do I create an op amp equivalent circuit?

An op amp equivalent circuit can be created by understanding the ideal characteristics of an op amp and using them to replace the non-ideal components in a real op amp circuit. This can be done through mathematical analysis or through simulation software.

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