FB - T.H. Lee EE214 Feedback Systems 1.0 Introduction A...

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T.H. Lee EE214 Feedback Systems ©Thomas H. Lee, rev. October 17, 2004; All rights reserved Page 1 of 34 Feedback Systems 1.0 Introduction A solid understanding of feedback is critical to good circuit design, yet many practicing engineers have at best a tenuous grasp of this important subject. We agree with MIT Pro- fessor Jim Roberge’s feeling that “feedback is so fundamentally important that analog engineers who don’t understand it should be legally barred from circuit design.” This chapter is intended as but a brief overview of the foundations of classical control theory, that is, the study of feedback in single-input, single-output, time-invariant, linear continu- ous-time systems. As usual, we’ll start with a little history to put this subject in its proper context. 2.0 A Brief History of Modern Feedback Although application of feedback concepts is very ancient (Og annoy tiger, tiger eat Og), mathematical treatments of the subject are a recent development. Maxwell himself offered the first detailed stability analyses, in a paper on the stability of the rings of Saturn (for which he won his first mathematical prize 1 ), and a later one on the stability of flyball-gov- ernor controlled steam engines (“On Governors,” Proc. Royal Soc. , no. 100, 1868). The first conscious application of feedback principles in electronics was apparently by rocket pioneer Robert Goddard in 1912, in a vacuum tube oscillator which employed pos- itive feedback. 2 As far as is known, however, his patent application was his only writing on the subject (he was more than a little preoccupied with rocketry, after all), and his con- temporaries were largely ignorant of his work in this field. 2.1 Armstrong and the Regenerative Amplifier Edwin Howard Armstrong’s 1915 paper 3 on vacuum tubes contained the first published explanation of how positive feedback (which he called regeneration) could be used to increase greatly the voltage gain of amplifiers. Although engineers today have a prejudice against positive feedback, progress in electronics in those early years was largely made possible by Armstrong’s regenerative amplifier, since there was no other economical way to obtain large amounts of gain from the primitive (and expensive) vacuum tubes of the day. 4 1. The Adams Prize, which he won in 1857. Maxwell argued that stability of the rings was possible only if they consisted of small particles. We now know from direct observation by Voyager that Maxwell was right. 2. U.S. Patent #1,159,209, filed 1 August 1912, granted 2 November 1915. 3. “Some Recent Developments in the Audion Receiver,” Proceedings of the IRE , v.3, pp. 215-247, 1915. 4. The effective internal “ g m r o ” of vacuum tubes back then was only on the order of five, so the gain per stage was typically quite low, requiring many expensive stages if conventional topologies were used.
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T.H. Lee EE214 Feedback Systems ©Thomas H. Lee, rev. October 17, 2004; All rights reserved Page 2 of 34 We can appreciate the essential features of Armstrong’s amplifier by examining the fol- lowing block diagram: FIGURE 1. Positive feedback amplifier block diagram
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This note was uploaded on 04/17/2008 for the course EE 214 taught by Professor Murmann,b during the Fall '04 term at Stanford.

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FB - T.H. Lee EE214 Feedback Systems 1.0 Introduction A...

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