Common mode stability in fully differential voltage feedback CMOS amplifiers

Abstract

In this study consideration is made of the common mode (CM) behavior of two-stage fully differential amplifiers in voltage feedback (FB) connection. In particular, the interaction between internal CMFB circuitry and the external feedback network is investigated in order to highlight a possible operating point instability condition, due to the presence of a positive feedback loop. Two alternative CMFB topologies are compared, one of which is affected by instability, with development of constraints on the feedback network parameters to be fulfilled in order to avoid instability. Comparison was made by circuit simulations with reference to a low voltage, deep sub-micron CMOS technology (0.12 /spl mu/m) implementation of a differential amplifier.