Opamp circuit for Vout = 2v1 - 3v2

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Discussion Overview

The discussion revolves around designing an operational amplifier (opamp) circuit to achieve a specific output voltage equation, Vout = 2v1 - 3v2. Participants explore the circuit design, focusing on the configuration of resistors and the use of a single opamp to meet the requirements. The conversation includes elements of homework-related problem-solving and technical reasoning.

Discussion Character

  • Homework-related
  • Technical explanation
  • Exploratory reasoning

Main Points Raised

  • One participant expresses confusion about whether the circuit requires two opamps with specified gains, suggesting a misunderstanding of the problem.
  • Another participant clarifies that a single opamp circuit is needed and proposes designing the negative section first, using the formula Vo = -(Rf/Ri)Vi, and then adjusting the positive gain with a voltage divider.
  • Participants discuss resistor values, with one suggesting that for v2, R2 should be three times R1, leading to questions about how to set all resistors to the same value.
  • There is a mention of needing a voltage divider at the positive input to achieve an overall gain of 2, with participants questioning the choices of resistors for this divider.
  • One participant introduces the concept of superposition to find the output of the circuit, indicating that the total output can be calculated by considering each voltage source separately.
  • Another participant notes that 'turning off' a voltage source means setting it to zero volts rather than leaving it open, which is important for the calculations.
  • There is a discussion about offset current compensation, with some participants indicating they have not covered it yet and expressing uncertainty about its implications for resistor configurations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the specific resistor values or configurations, and there are varying interpretations of how to approach the circuit design. Some participants agree on the need for a single opamp and the use of superposition, while others express confusion about the concepts involved.

Contextual Notes

Participants mention the need for specific resistor values and configurations but do not provide a complete resolution to the calculations or the implications of offset current compensation. There is also uncertainty about the definitions and applications of concepts like voltage dividers.

Squizzel
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Homework Statement



Design an Opamp circuit for which the output voltage will be of the following form

Vout = 2v1 - 3v2

Check with Multisim



Homework Equations





The Attempt at a Solution



We have only learned the basics of Opamps thus far, so I am confused with the question. This is asking for 2 opamps with the first one a gain of 5 and then the second with a gain of -3?

So I would calculate the first opamp with a gain of 5. would that mean that V2 is 5x what ever v1 is?
 
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No, they want a single op amp circuit.

Hint: design the negative section first: Vo = -(Rf/Ri)Vi, then add a voltage divider to the + input to reduce the + gain to 2.
 
Ok so for the v2 I got -R2/R1(v2) so that would be -3/1 so for v1 I get 1+3/1 * R4/R3+R4

So that means it needs to be 1/2 so all of the Rs must be 1?
 
Squizzel said:
Ok so for the v2 I got -R2/R1(v2) so that would be -3/1
So R2 = 3R1, right? So how can all the R's be the same?


so for v1 I get 1+3/1 * R4/R3+R4

So that means it needs to be 1/2 so all of the Rs must be 1?

For the + input pin you'd get 3+1 = 4 as you state. So what voltage divider do you put at the + input pin to get an overall + gain of 2? You gave the answer: 1/2. So what are your choices of the voltage divider resistors?

Question - did you cover offset current compensation yet?
 
rude man said:
For the + input pin you'd get 3+1 = 4 as you state. So what voltage divider do you put at the + input pin to get an overall + gain of 2? You gave the answer: 1/2. So what are your choices of the voltage divider resistors?

Question - did you cover offset current compensation yet?

We haven't gone over what offset current compensation is actually. I am also not sure what exactly a voltage divider is. But as to the question, I meant that the R3 and R4 would be 1k, not all of them.

Are you supposed to turn Vs2 off to calculate Vs1?
 
Squizzel said:
Are you supposed to turn Vs2 off to calculate Vs1?

The output of a linear circuit can be found with superposition.

Total output = output due to Vs2 with Vs1 zero + output due to Vs1 with Vs2 off.

That's why you can place all of this on one op amp.
 
Squizzel said:
We haven't gone over what offset current compensation is actually. I am also not sure what exactly a voltage divider is. But as to the question, I meant that the R3 and R4 would be 1k, not all of them.

Are you supposed to turn Vs2 off to calculate Vs1?
Yes. But 'turning off' means zero volts, not an open input!

Your choice of 1K for R3 and R4 is OK. When you get to offset current compensation you will find that R1||R2 should = R3||R4. You can do this and still have R1 = R2.
 

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