How do you design a differential amplifier using BJTs and what determines CMRR?

A BJT differential pair uses two matched transistors with emitters connected to a common current source. Differential input is applied between bases, and output is taken differentially or single-ended. CMRR depends on: tail current source impedance (higher is better), transistor matching, resistor matching, and temperature stability. Using a current mirror as tail current source instead of a resistor dramatically improves CMRR from 40-60dB to over 80dB.