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I have been dedicating my time to the study of Operational Amplifiers for the past few weeks.

I understood that an amplifier in an open loop configuration has a very high gain, in the case \$A \to \infty\$, so the output \$V_\text{out} = A(v^+-v^-)\$ is always \$v_{cc}\$ (this value being the OpAmp's power supply), therefore being always saturated, that is, a non-linear region with interesting applications in comparators.

On the other hand, the feedback can be done in two ways. The negative one, according to the more stable literature, I opened the equations in the configurations of inverting, non-inverting, summing, subtracting amplifier, and I also checked the virtual short circuit issue. I understood that the gain is controlled, and that's fine up to that point.

When I wondered about the positive feedback, I realized that the expressions were not changed much. And in the overwhelming majority of discussions about it, it is simply said that the OpAmp in the positive feedback configuration is saturated, always. And then you go from there to the applications (hysteresis comparator, oscillators, ...), but, my question is, mathematically, why is the OpAmp always saturated in a positive feedback configuration?

I saw a qualitative explanation, that negative feedback connects the output of the inverter input, and from the expression \$V_\text{out} = A(v^+-v^-)\$ , you can see that the output signal is decreased, while in positive feedback the signal is even more unstable (increased), but I wanted to know more details about this.

To make matters worse, I did a simulation in multisim, and the result left me even more confused. Below.

enter image description here

I tested another simulator here, and the result was also the opposite of what I expected. See below.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ What is your supply voltage for the op amp? Change the supply to see if the gain is supply dependent. \$\endgroup\$
    – sarthak
    Sep 21 at 2:49
  • \$\begingroup\$ The supply voltage is \$\pm 12~V\$. It is really at gain 10. \$\endgroup\$ Sep 22 at 4:15

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