[S1] In open loop mode, an OP-amp lacks Rf so it has no feedback, causing very high, often unstable (either positive/negative depending on respective input) gain. It's best used for comparators, etc. That’s why we almost never use them in open‑loop. Close loop controls the gain and make a circuit stable and predictable. [2] An OP amp can be an ideal amp, if used at infinite gain and bandwidth if used in this mode via usual DC gains of over 100,000 or 100dB.
So in this mode, an OP amp amplifies the contrast between inputs, but the gain is very high and unstable.
In close loop mode, an OP amp uses Rf to use either negative feedback (if connected to the inverting input) or positive feedback (if connected to the non-inverting input).
Using negative feedback can significantly reduce gain, thus stable predictable linear amplification for general amplifier circuits.
Using positive feedback reinforces the input signal IO reducing it, often making the amp to act as a comparator/oscillator, not a linear amplifier.
Closing an OP amp in open-loop by adding a Rf (between an OP amp's output and an input) greatly reduces and controls the close-loop gain.
So an OP amp in this mode amplifies the difference and stabilizes the gain to a controlled, usable value.
