Distortion Modeling of Feedback Two-Stage Amplifier Compensated With Miller Capacitor and Nulling Resistor

Accurate distortion modeling of feedback amplifier with Miller compensation is the main subject of this paper. Inspired by traditional Rosenstark method, a nonlinear signal-flow-based method has been developed. Based on the proposed method, the nonlinear coefficients of each gain stage can be evaluated exactly and the rules of cascading, feedback can be found conveniently. As a result, the second- and third-order harmonic distortion factors are obtained in a simple manner. The application range includes all kinds of Miller compensation strategies with or without feedforward compensation path. The distortion modeling of a two-stage opamp compensated with Miller capacitor and nulling resistor is used to show versatility of the proposed method. In addition, simplified distortion factor expressions, approximate characteristic frequencies of the distortion factors and the value of external configuration resistors for canceling the second-order harmonic components at DC are also derived and can be used to accelerate distortion evaluation of opamp designs. Accuracy of the method was verified by comparing the results between theoretical analysis and transistor level simulation.