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OPAMP - T.H Lee EE214 IC Op-Amps Through the Ages 1.0...

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T.H Lee EE214 IC Op-Amps Through the Ages ©2000 Thomas H. Lee, rev. October 17, 2004; All rights reserved Page 1 of 34 IC Op-Amps Through the Ages 1.0 Introduction The operational amplifier concept emerged from extensive development of electronic ana- log computers in the 1940s. Operational amplifiers get their name from their ability to per- form mathematical operations such as summation, integration and differentiation. With the addition of logarithmic amplifiers and feedback, one may perform multiplication and division as well. These abilities allow op-amp circuits to simulate differential equations, such as those describing the trajectory of an aircraft, for example. Countless analog com- puters were used throughout the second world war to produce “smarter” weapons capable of predicting where a target would be some time in the future, to determine the optimal fir- ing conditions to ensure with high probability that a shell would intercept it then. The abil- ity to “program” an analog computer rapidly by changing amplifier gains and connectivity made it an indispensable tool for both weaponry and academic research. Mechanical ana- log computers had preceded electronic ones, and changing gears and linkages made repro- gramming a cumbersome affair. The desire for compactness and universality led to the development of general-purpose high gain blocks intended for use within a feedback loop, the op-amp idea that is “obvi- ous” today. Perhaps the most famous op-amp of the vacuum tube era was the Philbrick K2-W, whose particularly elegant design served as the inspiration for transistorized coun- terparts throughout the 1950s and 1960s. Early efforts at integrated circuit op-amps attempted to replicate on a one-to-one basis the connectivity of earlier discrete designs. These pioneering efforts led to impractical results, for the reasons we’ve discussed numerous times: IC technology offers a different set of strengths and weaknesses, and these different rules (should) result in different design approaches. Early IC op-amps, practical or not, were first implemented in bipolar technol- ogies, and it is in those technologies that many now-familiar topologies debuted. The col- lection of op-amp schematics here traces that history in order to gain an appreciation for why IC op-amp topologies are what they are. Even though these are mainly bipolar exam- ples, a great many of these topologies are perfectly amenable to implementation in CMOS form as well, with appropriate modifications (e.g., for biasing, etc.). At the very least, these examples may serve as a valuable source of inspiration for new designs. The dates that follow the part numbers correspond to the first year of publication of a paper or data sheet, or the first year of product shipment.
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