Negative Feedback System and Circuit Design

Abstract

The negative feedback amplifier structure using an ideal integrator is derived. The time domain and frequency domain descriptions of the integrator are discussed. The response of the negative feedback amplifier in the time and frequency domains is analyzed. From these general conclusions are drawn about the behavior of negative feedback amplifiers. The ideal integrator is realized using controlled sources and passive elements. This realization clearly shows the cause for finite dc gain in real opamps. The effects of finite dc gain are analyzed. Relationships between amplifier specifications such as speed and accuracy and opamp parameters such as unity gain frequency and dc gain are derived. Methods of increasing the dc gain to improve accuracy are discussed. These lead to multistage amplifiers. The response of such systems in time and frequency domains are analyzed. It is shown that multistage amplifiers are potentially unstable. Stability conditions for negative feedback systems are discussed. The gain around the negative feedback loop is computed. The significance of loop gain is illustrated. Stability criteria related to the loop gain such as phase margin and Nyquist’s criterion are discussed. Frequently used criteria such as phase margin are clarified. Multistage amplifiers are essential for realizing high accuracy. Different techniques of realizing high gains while retaining stability-increasing the output resistance, miller compensation, and feedforward compensation are shown. There are various opamp architectures: folded/telescopic cascode; two stage miller compensated; feedforward compensated; and three stage. The design procedures for the two stage miller compensated opamp, the feedforward compensated opamp, and the three stage opamp are shown. These opamps will be compared in terms of their performance parameters-bandwidth, noise, power dissipation, slew rate, output swing. The design of a 350MHz bandwidth continuous-time active-RC filter using feedforward compensated opamps is shown. Measurement results from chips designed at IIT Madras illustrate the benefits of the feed-forward opamp architecture for low power applications. The design details of a three stage opamp with a DC gain exceeding 100dB is shown. The constraints on the design of different stages are evaluated. Simulation results of the opamp illustrate its suitability for a high precision application.