Op-Amp Design for Vo = 12VS1+3VS2 with 50 kΩ and 25 kΩ Input Resistances

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Discussion Overview

The discussion revolves around designing an operational amplifier (op-amp) circuit that achieves the output equation Vo = 12VS1 + 3VS2 while maintaining specific input resistances of 50 kΩ for VS1 and 25 kΩ for VS2. The focus is on exploring suitable op-amp configurations and addressing the challenges associated with the design.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses difficulty in finding a suitable op-amp model to produce the desired output, stating that neither a summation amplifier nor a difference amplifier seems appropriate.
  • Another participant questions the basis for dismissing the summation and difference amplifiers, prompting a discussion on the equations governing their gains.
  • Concerns are raised about the inability of the equations from the referenced article to satisfy the output requirements given the specified input resistances.
  • A suggestion is made that a summing amplifier can be configured as non-inverting, which could potentially address the design challenge.
  • Participants discuss the implications of the output equation, with one suggesting that a difference amplifier may not be suitable due to the absence of subtraction in the output expression.
  • There is a proposal to combine a summing amplifier with another type of amplifier, though concerns about the feasibility of achieving the required gains with the specified input resistances are reiterated.
  • One participant confirms that multiple op-amps can be used, which may simplify the design process by allowing unique feedback resistances for each op-amp.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the appropriate op-amp configuration. There are competing views on the suitability of summation and difference amplifiers, and the discussion remains unresolved regarding the best approach to meet the design requirements.

Contextual Notes

Participants express uncertainty about the relationships between feedback resistances and input resistances, indicating that the equations governing these relationships may not yield a viable solution for the design. The discussion highlights the complexity of achieving the desired output while adhering to the specified input resistance constraints.

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Homework Statement


Design an op-amp circuit that can perform the operation:Vo = 12VS1+3VS2, while simultaneously presenting an input resistance of 50 kΩ on the input side for source VS1 and an input resistance of 25 kΩ on the input side for source VS2.

Homework Equations





The Attempt at a Solution


I am looking at Op-Amp gains, and I cannot find a model that will produce this output. A summation amplifier won't do it, nor a difference amplifier. What kind of model should I use?
 
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Maylis said:
A summation amplifier won't do it, nor a difference amplifier.

On what basis did you get this?
 
A summing amplifier can be non-inverting.
 
Maylis said:
None of the equations for any of the op-amps in that article seem to work with what I want.

The output is (12)V1+(3)V2. Seeing this what op-amp configuration do you think would be suitable? Forget about gains for a second.
 
A Difference amplifier
 
Maylis said:
A Difference amplifier

"Difference" implies subtraction. Where do you see a subtraction?
 
Try to use summing amplifier + ?? amplifier
 
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the problem with the summing circuit is that Rf/50kΩ = 12 and Rf/25kΩ = 3 will not give a solution for Rf.

A non-inverting summer will have the same problem, where G is not satisfied by the two equations with the input resistances being set.
 
  • #10
Does your problem put a limit on to the number of amps used?
 
  • #11
No, you can use multiple op-amps
 
  • #12
So you can use summing amplifier and one voltage follower + 50K resistor between follower input the Vs1, and one non-inverting amplifier.
 
  • #13
Maylis said:
the problem with the summing circuit is that Rf/50kΩ = 12 and Rf/25kΩ = 3 will not give a solution for Rf.

A non-inverting summer will have the same problem, where G is not satisfied by the two equations with the input resistances being set.
Can you pass an input through a potential divider?
 
  • #14
Maylis said:
No, you can use multiple op-amps
Then it should not be difficult. You can have a unique Rf for each op-amp.
 

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