2.1: INTRODUCTION
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A control system is a system that regulates an output variable with the objective of producing a given relationship between it and an input variable or of maintaining the output at a fixed value. In a feedback control system, at least part of the information used to change the output variable is derived from measurements performed on the output variable itself. This type of closed-loop control is often used in preference to open-loop control (where the system does not use output-variable information to influence its output) since feedback can reduce the sensitivity of the system to externally applied disturbances and to changes in system parameters. Familiar examples of feedback control systems include residential heating systems, most high-fidelity audio amplifiers, and the iris-retina combination that regulates light entering the eye.
There are a variety of textbooks(G. S. Brown and D. P. Cambell, Principles of Servomechanisms, Wiley, New York, 1948; J. G. Truxal, Automatic Feedback Control System Synthesis, McGraw-Hill, New York, 1955; H. Chestnut and R. W. Mayer, Servomechanisms and Regulating System Design, Vol. 1, 2ndEd., Wiley, NewYork, 1959; R.N.Clark, Introduction to Automatic Control Systems, Wiley, New York, 1962; J. J. D'Azzo and C. H. Houpis, Feedback Control System Analysis and Synthesis, 2nd Ed., McGraw-Hill, New York, 1966; B. C. Kuo, Automatic Control Systems, 2nd Ed., Prentice-Hall, Englewood Cliffs, New Jersey, 1967; K. Ogata, Modern Control Engineering, Prentice-Hall, Englewood Cliffs, New Jersey, 1970.) available that provide detailed treatment on servomechanisms, or feedback control systems where at least one of the variables is a mechanical quantity. The emphasis in this presentation is on feedback amplifiers in general, with particular attention given to feedback connections which include operational amplifiers.
The operational amplifier is a component that is used almost exclusively in feedback connections; therefore a detailed knowledge of the behavior of feedback systems is necessary to obtain maximum performance from these amplifiers. For example, the open-loop transfer function of many operational amplifiers can be easily and predictably modified by means of external components. The choice of the open-loop transfer function used for a particular application must be based on feedback principles.
