Editing Differential amplifier (section)

==== Biasing ====

In contrast with classic amplifying stages that are [[Bipolar transistor biasing#Fixed bias (base bias)|biased from the side of the base]] (and so they are highly β-dependent), the differential pair is directly biased from the side of the emitters by sinking/injecting the total quiescent current. The series negative feedback (the emitter degeneration) makes the transistors act as voltage stabilizers; it forces them to adjust their V<sub>BE</sub> voltages (base currents) to pass the quiescent current through their collector-emitter junctions.<ref group="nb">Interestingly, it is as though the negative feedback has reversed the transistor behavior - the collector current has become an input quantity while the base current serves as an output one.</ref> So, due to the negative feedback, the quiescent current depends only slightly on the transistor's β.

The biasing base currents needed to evoke the quiescent collector currents usually come from the ground, pass through the input sources and enter the bases. So, the sources have to be galvanic (DC) to ensure paths for the biasing current and low resistive enough to not create significant voltage drops across them. Otherwise, additional DC elements should be connected between the bases and the ground (or the positive power supply).