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A simple method to guarantee absolute stability in multibit sigma--delta modulators (SDMs) is to ensure that the quantizer never overloads. This applies to any SDM. Derivation of the requirements for nonoverload have previously been shown for different types of modulators; the sigma--delta or output-feedback modulator with rounding quantizer as well as the error-feedback modulator using truncation. Here, these nonoverload requirements will be clarified and a unified formulation is presented that is not limited with regard to modulator topology or quantizer function. Index Terms--Data conversion, delta--sigma, high order, overload, sigma--delta, stability.

Nonlinear distortion in single-, two-, and three-stage operational amplifiers (opamps) is the main scope of this paper. For each opamp, distortion contributions from different groups of transistors are identified and plotted versus frequency. This makes it possible to find the strongest sources of distortion in the various frequency regions. Further, equations that describe the third harmonic as a function of circuit parameters and input frequency are presented. Despite the simplifications, these equations describe the third harmonic accurately. Further, they provide insight and understanding by connecting distortion to circuit parameters such as transconductances, capacitances, poles, and zeros. The comparison of the opamps shows that each opamp has a frequency region where the distortion is lower than for the other two. The three-stage opamp has far lower distortion at low frequencies, the single-stage opamp is better at high frequency and the two-stage opamp is best for the mid frequency range. Index Terms--Analog integrated circuits, feedback amplifiers, harmonic distortion, nonlinear distortion, operational amplifiers.