The performance of mixed-signal circuits are generally determined by the characteristics of active components. Analog cells used inside the chip have to drive well-deﬁned loads that are often purely capacitive, while the output buﬀer may demonstrate the capability of driving variable resistive and capacitive loads with low distortion. For this reason, the choice of the architecture and the design approach are mainly determined by the function to be realized. Furthermore, the realization of an ampliﬁer, which features both high dc gain and bandwidth, can lead to conﬂicting demands. The high gain requirement is met by multistage designs with long-channel transistors biased at a low current level, while the high bandwidth speciﬁcation is achieved in single-stage structures using short-channel transistors biased by a high-level current source.