OPA2991-Q1: Voltage simulation

I do not know if the model correctly simulates the offset voltage, but this is not a circuit that you should use (in real life, or in a simulation).

Without negative feedback, it works as a comparator, but two equal voltages cannot be compared.

What is the actual problem you're trying to solve?

Hello Uchihara,

An op amp will always try to drive the inputs equal to each other. This is usually done in negative feedback, as stated above by Clemens. In negative feedback without feedback components, the op amp output is able to drive an output voltage that is equal to the input voltage.

In the current setup, both inputs are forced to the same voltage, which means the output theoretically should not change. Based on the SPICE simulation initial conditions, this could be 0, it could be some other voltage too. In this case, since the op amp has offset voltage, the output drives to one rail or the other with as much output voltage as it can. If it were in feedback, it would be trying to drive the output to 13V+Vos. Since there is no feedback, and both inputs are being driven to 13V, the op amp will determine if it will drive to V+ or V- depending on the direction of the offset voltage.

Clemens is also correct that this is not a useful circuit. What goal are you trying to achieve from your circuit?

Best,
Jerry