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

[gfd43tg]

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?

 

[PhysicoRaj]

A summation amplifier won't do it, nor a difference amplifier.

On what basis did you get this?

 

[gfd43tg]

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.

 

[CWatters]

A summing amplifier can be non-inverting.

 

[PhysicoRaj]

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.

 

[gfd43tg]

A Difference amplifier

 

[berkeman]

A Difference amplifier

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

 

[Jony130]

Try to use summing amplifier + ?? amplifier

 

[gfd43tg]

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.

 

[PhysicoRaj]

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

 

[gfd43tg]

No, you can use multiple op-amps

 

[Jony130]

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

 

[NascentOxygen]

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?

 

[PhysicoRaj]

No, you can use multiple op-amps

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