Inverting Op Amp Homework - Solve V(output) Equation

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Discussion Overview

The discussion revolves around solving for the output voltage equation of an inverting operational amplifier (op-amp) circuit. Participants explore the implications of additional components in the circuit and seek guidance on their calculations and assumptions.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant states that the equation V(output) = V(input)(R2/R1) is valid only if certain components (R3, R4, and the Earth wire) are not present in the circuit.
  • Another participant suggests determining the voltage at the inverting input of the op-amp as a potential step in solving the problem.
  • A participant initially calculates a current but later corrects themselves, indicating a misunderstanding of their earlier calculation.
  • One participant proposes using Kirchhoff's Current Law (KCL) at the junction of R2, R4, and R3 to relate the voltage at that node to the input voltage, assuming ideal op-amp conditions.
  • Another participant mentions a specific current value (100 µA) flowing into a junction and questions the direction of that current flow.

Areas of Agreement / Disagreement

The discussion includes multiple viewpoints and approaches to solving the problem, with no consensus reached on the correct method or solution. Participants are exploring different aspects of the circuit without a clear resolution.

Contextual Notes

Participants have not fully clarified the assumptions regarding ideal op-amps, and there are unresolved aspects related to the circuit configuration and the impact of additional components on the output voltage calculation.

Who May Find This Useful

Students and individuals studying electrical engineering or circuit analysis, particularly those interested in operational amplifier circuits and their applications.

Lonesong
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Homework Statement



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Homework Equations



V(output) = V(input)(R2/R1)

The Attempt at a Solution


The equation above is valid if the back part of the upper circuit (involving R3,R4 and the Earth wire) does not exist. I tried to improvise with it by using the equation V(output) = V(input)(R2+1/(1/R3+1/R4))/R1 however the answer is wrong. Just need some guidance as to what I am doing wrong.
 
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Can you get the voltage at the inverting (-) input of the opamp?
 
That would be Vi/30 000= 3/30000 = 0.0001

edit: Oh wait, that's the current. Sorry.
 
Last edited:
Good, that's correct. So what is the voltage then?
 
If you assume the things that you need to about ideal op-amps, then you basically just have a simple circuit to solve with one voltage source and a few resistors. I find it helpful to apply KCL at the node where R2, R4, and R3 all join. You can relate the voltage at this node to the input voltage, because you know what the voltage at the inverting input must be.
 
You have 100 uA flowing into the junction of R1, R2 and the op amp - terminal. Where is that current flowing to from there?
 

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