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So we are given the equation:

Vout=Rf/Rs(V2-V1)

I'm just wondering how would we solve it when the Rf/Rs are different values like they are in this circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

Would we just use nodal analysis after the 5V source @ Va?

So then formula would be: Vout=(36k/12k)(Va-10V)?

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  • \$\begingroup\$ The first google hit for 'differential amplifier derivation' shows the correct analysis. \$\endgroup\$
    – user133493
    Apr 26, 2017 at 1:17
  • \$\begingroup\$ So if I am looking at the same one I am wrong. Should be (36k/12k)(5-10) \$\endgroup\$
    – noreturn
    Apr 26, 2017 at 1:20
  • \$\begingroup\$ Wrong. It is not that simple, check again. \$\endgroup\$
    – user133493
    Apr 26, 2017 at 1:36

1 Answer 1

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There are a lot of ways to solve this. If you assume that the TL081 is ideal, you can take some short cuts. If it is ideal, the current into the inverting and non-inverting inputs is 0. For the non-inverting input, the voltage is just 5*30/44. You recognize this because it is just a voltage divider. Now you know the feedback is going to force the inverting input to have the same voltage, which is 3.41V. So the current through R1 is (10-3.41)/12k. But this same current has to flow through R2. Since the current into the non-inverting is 0. So now you know the voltage across R2 and the voltage at the non-inverting input. You can take it from here. Understanding what is going on will help you more than formulas.

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  • \$\begingroup\$ How do we know to divide by 12k and not any others? So would the (10-3.41)/12 give the answer they would most likely ask for? \$\endgroup\$
    – noreturn
    Apr 27, 2017 at 3:03

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