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:
simulate this circuit – Schematic created using CircuitLab
Also the source voltage is +14v/-14V if that helps.
And here are my calculation :
- A=Vin/Vout = -R2/R1
- VinR1 =-R2Vout
- -VinR1/R2 = Vout
- -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.
Best Answer
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.