Which internal component of an operational amplifier (op-amp) is primarily responsible for increasing the input signal's amplitude before it reaches the output stage?

The part that first boosts the signal is the input differential amplifier, usually a pair of transistors that form a differential pair. It takes the tiny voltage difference between the non‑inverting and inverting inputs and amplifies it with high open‑loop gain. This amplified voltage then drives the next stages of the op‑amp, which further increase the signal before it reaches the output driver. For example, if the differential pair has a gain of 10 000, a 1 mV input difference becomes 10 V at its output, ready to be fed to the rest of the circuit. This early amplification is what lets the op‑amp provide very high overall gain.