Engineering Opamp circuit for Vout = 2v1 - 3v2

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The discussion centers on designing an op-amp circuit to achieve the output voltage equation Vout = 2v1 - 3v2. Participants express confusion about using two op-amps versus a single op-amp configuration, with hints provided for designing the circuit. Key points include calculating gains for the op-amps and using a voltage divider to achieve the desired gain at the positive input. The concept of superposition is introduced for analyzing the circuit, emphasizing the need to turn off one source when calculating the effect of the other. Overall, the conversation highlights the challenges of applying basic op-amp principles to meet specific output requirements.
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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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