Introduction to CMOS OP-AMPs and Comparators

CHAPTER 5

COMPARATORS

Comparators are the second most widely used components in electronic circuits, after operational amplifiers. A voltage comparator is a circuit that compares the instantaneous value of an input signal ν_in(t) with a reference voltage V_ref and produces a logic output level depending on whether the input is larger or smaller than the reference level. The most important application for a high-speed voltage comparator occurs in an analog-to-digital converter system. In fact, the conversion speed is limited by the decision-making response time of the comparator. Other systems may also require voltage comparison, such as zero-crossing detectors, peak detectors, and full-wave rectifiers. In this chapter a number of approaches to comparator design are presented. First, the single-ended auto-zeroing comparator is examined, followed by simple and multistage differential comparators, regenerative comparators, and fully differential comparators. Several design principles are introduced that can be used to minimize input offset voltage and clock-feedthrough effects.

5.1. CIRCUIT MODELING OF A COMPARATOR

One very important and widely used comparator configuration is a high-gain differential input, single-ended output amplifier [1,2], Figure 5.1a shows the symbol of a differential comparator which is very similar to that of an operational amplifier. Usually, the comparator stage is followed by a latch, which is essentially a bistable multivibrator. The latch provides a large ...