Re: Op-Amp
The difference between the + and - pins of the opamp is ideally 0. This is called the virtual short principle.
==> The virtual short principle is ONLY true when there is MORE negative feedback around the opamp than positive feedback.
For instance, in a basic inverting opamp, the negative feedback from the output pin is returned to the - terminal. There is NO positive feedback in this configuration because the + pin is connected to ground.
In other configurations, it is important to make sure that there is more negative feedback than positive feedback. If there is more positive feedback than negative feedback, then ALL bets are off in terms of the virtual short principle, and thus you can not be guaranteed that the + and - terminals are at the same potential.