Figure 2.1 illustrates the most general form of differential amplifier. Note that including
ground, it is a four-port circuit. Most practical DAs have only a single-ended output, Vo,
and are thus three-port circuits. The deflection amplifier circuits of most analog
oscilloscopes are an exception to this rule, preserving the differential circuit architecture
from the input to the output, to the CRT deflection plates.
In the analysis of DAs, it is expedient to define difference-mode and common-mode input
and output signals.