Track 1 Early Morning
Systematic Design of Bandgap Reference Circuit with Emphasis on Self-bias Loop Dynamics

Abstract

Sub-1V bandgap designs have recently become prominent candidates with the scaling of supply voltages and technology. The tutorial will brief the latest advances in the bandgap world with architectural descriptions and truly digital reverse bandgap design. Designing operational amplifiers for sub-1V bandgaps has become challenging for three primary reasons: the virtual node of the op-amps experiences a large swing, there is lower headroom for devices, and the bias current varies with temperature. Operational amplifier offset, particularly in bandgap designs, is critical as it gets amplified by the PTAT gain to the bandgap output. There are three primary types of offset: systematic offset, random offset, and random systematic offset. The only known solution for reducing systematic offset from the literature is self-bias, which replicates the bandgap current into the op-amp. However, the loop dynamics of self-bias, such as whether it operates as a positive or negative feedback loop, are seldom discussed in the literature. This tutorial will emphasize the impact of self-bias concerning DC gain and the negative feedback loop. The loop dynamics of self-bias are intriguing, as the feedback sign changes from negative to positive with frequency. This tutorial will present a detailed qualitative and quantitative analysis of the self-bias loop, providing design guidelines for implementing self-bias. The tutorial will brief you about the solution to random offset by chopping and describe the systematic analysis of the chopping technique specific to the bandgap world. Finally, the tutorial will conclude with the design of startup circuits. Known startup issues arise when the diodes carry extremely low currents, but there are also startup issues related to the maximum operational amplifier current introduced by the self-bias loop.