Because op-amps have very high gain when connected in an open-loop configuration, any noise and unwanted signals are gained up by the same factor as the so-called “wanted” signals. This makes open-loop configuration highly sensitive so it’s mainly used in comparators.
The closed-loop configuration, on the other hand, provides gain desensitization. For example, the closed-loop gain is now controlled by feedback resistors, which makes the gain less sensitive to device parameters and noise than the open-loop gain.
With the use of feedback resistors, we now can control the input and output impedances of an op-amp by either raising or lowering the input and output impedances in several feedback topologies. In addition, a closed-loop configuration in op-amps reduces nonlinear distortion in an amplifier making the output proportional to the input. This is contributed by the reduction in noise signal gain.
