What are the voltage ratios in an op amp circuit?

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Discussion Overview

The discussion revolves around determining the voltage ratios in an operational amplifier (op-amp) circuit, focusing on the algebraic analysis involved. Participants explore the implications of ideal op-amps and the role of specific resistors in the circuit.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant expresses difficulty with the algebra involved in finding the voltage ratios and seeks validation of their work.
  • Another participant notes that if the op-amps are ideal, they have infinite gain and do not draw current at their inputs, suggesting that R2 is not involved in the analysis and that R5 should also be disregarded.
  • A participant questions the simplicity of the analysis and recalls a caution from their professor about not applying Kirchhoff's Current Law (KCL) at the output of an op-amp, indicating a need to revisit their approach.
  • A later post indicates a revision of the initial attempt, with a participant claiming to have found and corrected an algebra error in their voltage ratio calculation.
  • Another participant reassures that the revised work is correct.

Areas of Agreement / Disagreement

Participants generally agree on the ideal characteristics of op-amps and the implications for the analysis, but there is uncertainty regarding the application of KCL at the output and the correctness of the algebraic manipulations. The discussion includes multiple attempts and revisions without a clear consensus on the final solution.

Contextual Notes

Participants have not fully resolved the implications of disregarding certain resistors in the analysis, and there may be missing assumptions regarding the circuit configuration and the application of KCL.

gfd43tg
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I have to find both of the voltage ratios. This is just ugly algebra. Here is what I came up with, and was wondering if I was correct.
Thanks

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If the opamps are ideal, they have infinite gain and their inputs draw no current.

You correctly saw that this means R2 is not involved in the analysis, but you failed to notice that R5 is not involved for the same reasons.

You'll need to redo your work with R5 replaced with a wire; no R5, in other words.
 
Is it really that easy?

ImageUploadedByPhysics Forums1399705402.263499.jpg


Edit: Wait a minute, I remember the professor saying to never do KCL at the output of an op-amp, I will redo this.
 
Ok here is my next attempt hopefully it's right

ImageUploadedByPhysics Forums1399706185.468647.jpg


ImageUploadedByPhysics Forums1399706199.284359.jpg
 
I found an algebra error in my Vout/Vin and fixed it

ImageUploadedByPhysics Forums1399709957.638376.jpg
 
You're good to go!
 

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