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I am trying to find Vout for the following circuit. Unfortunately, I do not get the right answer. What am I doing wrong? Here is my circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

Also the source voltage is +14v/-14V if that helps. And here are my calculation :

  1. A=Vin/Vout = -R2/R1
  2. VinR1 =-R2Vout
  3. -VinR1/R2 = Vout
  4. -5*1/4 = -1,2V

The problem is the answer is supposed to be Vout=25V. But I do not understand how to get to that answer. Any help would be greatly appreciated.

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  • \$\begingroup\$ Your gain equation is wrong. Do the KVL and find Vout directly by assuming that the voltage at inverting input is equal to Vin = 5V \$\endgroup\$
    – G36
    Feb 27, 2019 at 15:38
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    \$\begingroup\$ Also does not forget the output voltage cannot be larger than the supply voltage +14V/-14V. \$\endgroup\$
    – G36
    Feb 27, 2019 at 15:40
  • \$\begingroup\$ But the answer is supposed to be 25V according to the solution document does that mean that the solution document is wrong and that the answer is supposed to be 14v? \$\endgroup\$
    – J.C
    Feb 27, 2019 at 15:49
  • \$\begingroup\$ If the positive supply voltage is indeed equal to +14V then the op-amp output voltage cannot be higher than this voltage. Hence the correct answer is +14V if we ignore the op-amp positive saturation voltage. e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/05/08/… \$\endgroup\$
    – G36
    Feb 27, 2019 at 15:53
  • \$\begingroup\$ I think you are right I don't see how magically be more voltage it would go against all laws of conservation of energy. Thank you very much for your help. \$\endgroup\$
    – J.C
    Feb 27, 2019 at 15:59

1 Answer 1

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1) Op amp inputs are high-impedance. Therefore, the + input is at 5 volts.

2) Op amps exhibit very high gain. Therefore, if the op amp is being operated usefully, the - input is at 5 volts, since any output will produce a very small (usefully approximated as zero) voltage difference between + and - inputs.

3) The voltage across R1 will be 5 volts and the current through R1 will be 5 mA by Ohm's Law.

4) Op amp inputs are high-impedance. Therefore, the current through R2 will be 5 mA.

5) The voltage across R2 will be 20 volts by Ohm's Law.

6) The output voltage will be the sum of the voltages across R1 and R2, or 25 volts.

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  • \$\begingroup\$ @Junior Cortenbach - Please note that, in line with G36's comments, you will never get 25 volts out of an op amp with +/- 14 volt supplies. Therefore, item 2 will not hold. The op amp will be operating in saturation, and the - input will be at something like 2 volts. \$\endgroup\$ Feb 27, 2019 at 18:16

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