Generic operational amplifier for MCU integration

Abstract

The versatility and useability of the opamp makes it widely used in hardware design. Thus having a general purpose opamp built in to the MCU can save many hardware developers both time, effort and money. As Microchip's popular Atmel AVR line of MCUs doesn't yet have this as an option. \\

This thesis will attempt to design a general purpose opamp in Atmel's 59k91 130nm CMOS technology based on Microchip Atmel's specifications. The main specifications are: rail-to-rail input and output ($\pm$100mV on output), a GBW of 3MHz and an open loop gain of 90dB. This thesis discusses opamp design and the specifics of the chosen design. Because of the requirement for rail-to-rail operation on both inputs and on the output some special considerations had to be made. This design is a two stage amplifier using dual input stages with constant $g_{m}$ control by way of quadroupling the bias current when only one input pair is active, and a class AB output stage with floating bias. The first stage also incorporates a modified telescopic cascode to meet the requirement for open-loop gain. \\

The design meets all specification requirements in the nominal corner except for stability, and it falls short in some other corners. Further tweaking is thus necessary before layout and implementation. In addition, the design needs to be fitted with the switches needed for integration with the MCU power gating.