In a MOSFET-based differential amplifier, what is the primary purpose of using a current mirror as an active load?
An active load, typically implemented with a PMOS current mirror, provides a high impedance load to the NMOS differential pair. This high dynamic resistance (ro of the transistors) results in a much higher voltage gain (gm * ro) than what is achievable with a simple resistive load. Furthermore, the current mirror naturally converts the differential current signal from the input pair into a single-ended output voltage at its high-impedance node.
