Electronic – op-amp: what is internal frequency compensation

Assuming you've already studied the internal schematic of the classic 741 op-amp, you already know the major internal blocks are a differential-to-single-ended input stage (differential pair fed by current source), a common-emitter amplifier stage, and an output driver stage. The internal compensation is a small negative feedback capacitor within the common-emitter amplifier stage. If you refer to TI LM741 datasheet, 7.2 Functional Block Diagram, the internal compensation capacitor is C1 30pF near the center of the schematic. (Note: TI's block diagram has an error, they have two transistors labeled Q15 and one unlabeled transistor.)

The purpose of this internal compensation is to reduce the open-loop gain at higher frequency, so that there will be less than unity gain at the frequency where the phase shift becomes 180 degrees. This is a heuristic to help ensure that there is sufficient phase margin to avoid oscillating, when the op-amp is externally configured as a unity-gain amplifier.

This is also a marketing feature for the IC manufacturer. They can make one version of the op amp that has internal compensation, for customers who care about unity-gain configuration. Then they also make a slightly different version that does not have the internal compensation, and thus will not be unity-gain stable. However the "uncompensated" op-amp will be stable at some minimum gain, for example 2V/V or 5V/V -- this will be specified on the data sheet.