Op Amp Vout Calculation for Non-Inverting Configuration

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Discussion Overview

The discussion revolves around calculating the output voltage (Vout) for a non-inverting operational amplifier (op-amp) configuration, particularly focusing on a circuit that does not conform to the standard resistor feedback model. Participants explore the implications of feedback, ideal op-amp behavior, and the relationship between time and frequency domains in the context of this problem.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion regarding the application of the standard Vout equation for a non-inverting op-amp due to the absence of a resistor from the non-inverting terminal to the output.
  • Another participant suggests starting with ideal op-amp approximations and writing equations that relate node voltages, proposing a method of evaluating hypothetical input and output values to check for consistency with the amplifier equation.
  • A third participant assumes the circuit is a voltage amplifier and advises double-checking the schematic for uncommon features that may affect the analysis.
  • Another participant points out the presence of feedback to the negative input through a capacitor and resistor, emphasizing that negative feedback will equalize the op-amp inputs regardless of the feedback form.
  • One participant questions the familiarity of another with the concepts of impedance in relation to capacitors and resistors, hinting at the need for understanding the frequency domain implications.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are multiple competing views on how to approach the problem and the implications of the circuit configuration. The discussion remains unresolved regarding the correct application of the feedback and the resulting calculations.

Contextual Notes

Participants note potential limitations in understanding due to the complexity of the circuit and the need for clear schematic representation. There are also hints at unresolved mathematical relationships and assumptions regarding the circuit's behavior.

esob
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Homework Statement


Solve for Vout

Homework Equations


Vout=Vin(1+Rf/R)

The Attempt at a Solution


I get it is a non-inverting op amp but there is no resistor from the non-inverting terminal to the output.
I have no idea how the equation would be. I understand that noninverting is Vout=Vin(1+Rf/R) but this op amp doesn't match that form and I have never solved one like this.
 

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Have you covered the ideal op-amp approximations in your class yet? I would start with those and then write the equations that relate the node voltages via the amplifier function and the feedback relationship from the output to the inverting input, etc.
If the equations and the algebra aren't clear to you you could try guessing at some numbers and then evaluating what that implies for the other node voltages. For example, what if the input was 2V and the output was 4V, would the amplifier equation be satisfied? If not what adjustments would you make to make everything agree? Then what would that mean for the equations relating those node voltages?
 
BTW, you didn't say, but I'm assuming that this is a voltage amplifier.
Also, you should probably double-check that you have drawn your schematic correctly. There is something minor about your circuit that is uncommon in real world applications.
 
Hi esob,

Consider that there is feedback to the negative input of the op-amp via the parallel connection of C and Rf
(whatever that net impedance happens to be). There's negative feedback so that will drive the two op-amp inputs to the same potential. That's just the way (ideal) op-amps behave. Doesn't matter what the form of the feedback is; If it's there, it''ll get the job done and drive those inputs to the same value.

What can you say about the potential at the V+ input?

I'm not sure how much more to give away here without seeing some of your own work to get a feeling for your level of understanding of the material, so I'd like to hear (okay, see) your thoughts so far.
 
Esob - are you familiar with the interdependences between time and frequency domain?
Do you that a capacitor constitures something like a resistance for sinusoidal signals (called "impedance")?
Do you know the expression for such an impedance ?
Do you know how to combine an impedance with an ohmic resistor?
 

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