Calculating Output of Op-Amp in Given Circuit

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Discussion Overview

The discussion revolves around calculating the output of an operational amplifier (op-amp) in a given circuit, specifically focusing on the application of Kirchhoff's Current Law (KCL) and the characteristics of inverting amplifiers. The context is primarily homework-related, involving theoretical calculations and circuit analysis.

Discussion Character

  • Homework-related
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant presents an initial calculation using KCL, arriving at a potential output of Vo=20, but questions the use of a negative sign in their equation.
  • Another participant identifies the circuit as an adder or summer and provides a formula for the output, emphasizing the importance of negative signs in the calculations.
  • A subsequent post reiterates the output calculation, questioning the placement of the negative sign before the feedback resistor value.
  • Another participant clarifies that the negative sign is necessary because the input is connected to the inverting input of the op-amp.
  • A final post acknowledges the clarification and expresses gratitude for the input.

Areas of Agreement / Disagreement

Participants generally agree on the necessity of using negative signs in the calculations due to the inverting nature of the op-amp input. However, there is some uncertainty regarding the initial calculations and the interpretation of the circuit configuration.

Contextual Notes

There are unresolved aspects regarding the assumptions made in the calculations, particularly concerning the definitions of input types and the specific circuit configuration. The discussion does not fully resolve the initial confusion about the output value.

uzair_ha91
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Homework Statement


Calculate the output of the op-amp as shown in the given circuit.
http://img9.imageshack.us/img9/7554/184v.png
(Answer=0)

Homework Equations


Kirchhoff's Current Rule


The Attempt at a Solution



By applying KCL,
I1+I2=I3
V1/R1 + V2/R2 = Vo/R3
Substituing the values and rearranging the equation we get,
Vo= (5/10,000 + 2/4,000)*20,000
Vo= 20
But then I realized that if i use a minus sign between (5/10,000 + 2/4,000), I get Vo=0...
So does that minus sign come into the equation because we're trying to find out the output of an inverting amplifier, or is it simply because V2="-"2
 
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This type of circuit is known as adder or summer

AdderAmp.gif


The solution is given by:

vo = - RF * [ V1/R1 + V2/R2 + ... + Rn/Rn]

SOLUTION:

Vo = - 20k * [ 5/10k + -2/4k] ; I think you forgot to use negative signs
Vo = 0
 
Mr.Green said:
Vo = - 20k * [ 5/10k + -2/4k] ; I think you forgot to use negative signs
Vo = 0

Why did you put a minus sign before 20k?
 
Since the input is on the inverting input a negative sign should be used.
On yourpost you didn't specify the type of inputs of the op-amp, but it seems that the input is on the inverting input.
 
Yeah you're right...thanks
 

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