Chapter 6 - Feedback and Stability Theory

Abstract

The gain of all op amps decreases as frequency increases, and the decreasing gain results in decreasing accuracy, as the ideal op amp assumption breaks down. In most real op amps, the open loop gain starts to decrease before 10 Hz, so an understanding of feedback is required to predict the closed loop performance of the op amp. This chapter discusses feedback and stability theory. The real world application of op amps is feedback controlled and depends on op amp open loop gain at a given frequency. A designer must know theory to be able to predict the circuit response regardless of frequency or open loop gain. In a typical control system, after reference input move through the control elements and reach the end of the circuit, the final signal is evaluated by feed back elements and send it back to the summation point to provide feed back. While designing the circuit the idea is to reduce the diagram to its canonical form because the canonical feedback loop is the simplest form of a feedback loop. All feedback systems can be reduced to the canonical form, so all feedback systems can be analyzed with the same math. A canonical loop exists for each input to a feedback system; although the stability dynamics are independent of the input, the output results are input dependent. The response of each input of a multiple input feedback system can be analyzed separately and added through superposition.