Another frequently occurring analog building block is the differential pair.
The basic topology of an N-channel MOS differential pair is shown in
Figure 5.1. The differential pair consists of two identical transistors with connected source terminals.
A constant, quiescent current 2I is drawn from the common source node,
and the current is divided between the MOS transistors. How this current
is divided between the two transistors depends on the gate potentials.
The transistor with the highest gate potential conducts more current.
The basic function of the differential pair is to convert a voltage
difference, 
, between the input gate voltages to a current
difference, 
, between the output currents.
Figure 5.1: N-channel MOS differential pair.