By Yasir Sabir and Shabbir Majeed Chaudhry, University of Engineering & Technology, Pakistan

There’s no stopping the current needs for low-power analogue, digital and mixed-signal circuits, especially in portable devices. Furthermore, in nanometre devices, the constraints of low power with large voltage swings in low supply voltages call for the unorthodox design using multi-stage amplifiers. This is further accompanied by trends for high-data rates, continually rising bandwidths and high drive capabilities, leading to the design of output stages to drive large capacitive loads.

The class AB output stage is a primary contender for increasing speeds and output swings. With only a single stage, this amplifier achieves high gain by cascading, which may, however, lead to a low output swing because of the small supply voltages in nanometre CMOS technology. To maintain high gain and a high output swing would logically force the designer to apply a two- or three-stage fully-differential amplifier for doubling the output swing.