# Op-Amp Design

1. Mar 6, 2014

### Maylis

1. The problem statement, all variables and given/known data
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.

2. Relevant equations

3. 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?

2. Mar 6, 2014

### PhysicoRaj

On what basis did you get this?

3. Mar 6, 2014

### Maylis

By looking at the equations of the gains of the respective amplifiers.

The equation has a positive sign, and the equation for a difference amplifier for Vout is given here
http://en.wikipedia.org/wiki/Operat...erential_amplifier_.28difference_amplifier.29

Notice the -Rf/R1 * v1 term.

None of the equations for any of the op-amps in that article seem to work with what I want.

4. Mar 6, 2014

### CWatters

A summing amplifier can be non-inverting.

5. Mar 6, 2014

### PhysicoRaj

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.

6. Mar 6, 2014

### Maylis

A Difference amplifier

7. Mar 6, 2014

### Staff: Mentor

"Difference" implies subtraction. Where do you see a subtraction?

8. Mar 6, 2014

### Jony130

Try to use summing amplifier + ?? amplifier

9. Mar 7, 2014

### Maylis

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. Mar 7, 2014

### PhysicoRaj

Does your problem put a limit on to the number of amps used?

11. Mar 7, 2014

### Maylis

No, you can use multiple op-amps

12. Mar 7, 2014

### Jony130

So you can use summing amplifier and one voltage follower + 50K resistor between follower input the Vs1, and one non-inverting amplifier.

13. Mar 7, 2014

### Staff: Mentor

Can you pass an input through a potential divider?

14. Mar 8, 2014

### PhysicoRaj

Then it should not be difficult. You can have a unique Rf for each op-amp.

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