An operational amplifier has the internal compensation circuit for stability which limits™ its working bandwidth. Frequency response of the compensated Op Amp has slope of −6 dB/octave™ or −20 dB/decade™. Unity gain frequency defines the bandwidth where the Op Amp is able to amplify a signal™. If we multiply the gain and frequency at any point, the result™ is the same, allowing us to use this parameter to select™ the appropriate Op Amp. It is called™ Gain-Bandwidth Product, GBW or GBP. The limited open-loop gain introduces a closed™-loop gain and phase error.
But we want to optimize our circuits, right?