Integrated circuits comprise the core of essentially all electronic systems. In the design of many integrated circuits, one task of the design engineer is to provide accurate voltages to sub blocks in the circuit structure. The circuits that provide these voltages are often referred to as voltage references. A widely used class of voltage references that typically have low supply, process, and temperature sensitivities are bandgap references whose output voltage is dominated by the bandgap voltage of silicon. Though several structurally different bandgap reference circuits are widely used in industry, there is little in the literature that focuses on how the performance of these circuits can be optimized or how the performance of different bandgap circuits compare. The task of optimization and comparison is complicated by the realization that each of the bandgap circuits themselves have several degrees of freedom in the design. In this work, a metric for fairly comparing the basic performance of different bandgap references based upon the normalized second-order temperature derivative is introduced. This metric is used to compare the performance of several of the most popular bandgap reference circuits that are used in the production. The comparisons show that even though the structure of these reference circuits are fundamentally different and even though each circuit has several degrees of design freedom, the normalized temperature coefficients of all circuits in the comparison group at a fixed operating temperature are the same. The comparisons also show that the designer cannot optimize the basic performance of any of these circuits through judicious utilization of the degrees of design freedom.
In this work, a new very low power voltage reference obtained by replacing the diode-connected bipolar transistors in a basic bandgap circuit with diode-connected MOS transistors operating in deep weak inversion is also discussed. An analytical formulation of the weak-inversion MOS voltage reference shows that the MOSFET-based structure has even lower temperature sensitivity than the basic bandgap circuits. The issue of practicality of the MOS-based reference is, however, of concern since the extremely low currents appear to create the need for very large resistors which are not realistically available in most standard CMOS processes
