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Op Amp and Comparators. Op Amp Circuit Collect This story begins back in the vacuum tube era and continues until today While most of today’s op amp users are probably somewhat familiar with integrated circuit IC op amp history, considerably fewer are familiar with the non-IC solid-state op amp. And, even more likely, very few are familiar with the origins of dstasheet op amp in vacuum tube form, even if they are old enough to have used some of those devices in the 50’s or 60’s.
This chapter of the book addresses these issues, with a narrative of not only how op amps originated and evolved, but also what key factors gave rise to the op amp’s origin in the first place.
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dxtasheet Of these, there were two key inventions very early in the century. The first was not an amplifier, but a two-element vacuum tube-based rectifier, the “Fleming diode,” by J. Fleming, patented in see Reference 1.
This was an evolutionary step beyond Edison’s filament-based lamp, by virtue of the addition of a plate electrode, which when biased positively captured electrons emitted from the filament cathode. Since this device passed current in one direction only, it performed a rectification function. This patent was the culmination of Fleming’s earlier datasheeet in the late years of the 19th century.
Section H Op Amp History_图文_百度文库
A second development and one more germane to amplificationwas the invention of the three-element triode vacuum tube by Lee De Forest, the “Audion,” in This was the first active device capable of signal amplification see Reference 2. De Forest added a control grid electrode, between the diode filament and plate, and an amplifier device was born.
While these first tubes of the 20th century had their drawbacks, the world of modern electronics was being born, and more key developments were soon to follow. After World War II, there was then a transition period, as vacuum tube op amps were improved and refined, at datasehet in circuit terms.
But, these amplifiers were fundamentally large, bulky, power-hungry devices. Note— this chapter of the book is not necessarily required for the use of op amps, and can be optionally skipped. Nevertheless, it should offer interesting background reading, as it provides a greater appreciation of current devices once their beginnings are more fully understood. Once IC technology became widely established, things moved quickly through the latter of the 20th ad5547 years, with milestone after milestone of progress being made in device performance.
A Definition for the Fledgling Op Amp Although it may seem inappropriate to define what an op amp was in those early days at this point in the book, it is necessary to do so, albeit briefly. This is because what is commonly known today as an op amp is different in some regards from the very first op amps.
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The introductory section of Chapter 1, where the discussion is more closely oriented around today’s op amp definition, supplements the meaning below. The very first op amps were not even called such, nor were they even called “operational amplifiers”. The naming of the device came after the war years, in For this historical discussion, it may be more clear to call one of these first op amps a general-purpose, DC-coupled, high gain, inverting feedback amplifier.
This of course is a loose definition, but it nevertheless fits what ax547. General-purpose may be interpreted to mean that such an amplifier or multiple amplifiers operates on bipolar power supplies, with input and output signal ranges centered around datasbeet ground.
DC-coupled response implies that the signals handled include steady-state or DC potentials, as well as AC signals. Ad47 mode operation means that this feedback amplifier had, in effect, one signal input node, with the signal return being understood as ground or common.
Multiple signals were summed at this input through resistors, along with the feedback signal, via another resistor. Note that this single-ended operating mode is a major distinction from today’s differential input op amps. Operation of these first feedback amplifiers in datasheeet a single-ended mode was, in fact, destined to continue for many years before differential input operation became more widespread.
A feedback amplifier of this type could be used in a variety of ways, dependent upon the nature of vatasheet feedback element used with it. This capability of satisfying a variety of applications was later to give rise to the name.
So, given this background, op amp history can now be explored. The add547 of the feedback amplifier has to rank as one of the more notable developments 20th century— right up there with the automobile or airplane datazheet breadth of utility, and general value to engineering. And, most importantly, such feedback systems, although originally conceived as a solution to a communications problem, operate today in more diverse situations.
Today the application of negative feedback is so common that it is often taken for granted. But this wasn’t always the case. Black first developed feedback amplifier principles. Note that this was far from a brief inspirational effort, or narrow in scope.
In fact it took some nine years ad574 the broadly written patent application, until the issuance see Reference 3. Additionally, Black outlined the concepts in a Bell System Technical Journal article, and, much later, in a 50th anniversary piece where he described the overall timeline of these efforts see References 4 and 5.
But, like circumstances surrounding other key inventions, there were others working on negative feedback amplifier applications.
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One example would be Paul Voigt’s datashheet work see References 6 and 7. Philips in the Netherlands is said to have been exploring feedback amplifiers within roughly the same time frame as Black late twenties to early thirties. Tellegen published a paper on feedback amplifiers, with attributions to K. Posthumus datashest Black see References 9 and But, it isn’t the purpose here to challenge Black’s work, rather to note dtasheet sometimes overlapping but independent parallel developments occur, even for major inventions.
Other examples will be seen of this shortly, in the developments of differential amplifier techniques. In the long run, a broad-based, widely accepted body of work tends to be seen as the more significant effort. In the case of Black’s feedback amplifier, there is no doubt that it is xd547 most significant effort.
It is also both broad-based and widely accepted. There are also many earlier positive feedback uses; a summary is found in Reference Some suggest Paul Voigt as the true feedback amplifier inventor, not Black see Ref. Examination of Voigt’s UK patentfails to show a feedback amplifier theory comparable to Black’s detailed exposition of Ref. In fact, there are no equations presented to describe Voigt’s system behavior. Posthumus, apparently a practice with N. Philips UK patents of that period.
The patent does show a rudimentary feedback amplifier, but dataxheet the overall clarity is marred by various revisions and corrections, to both text and figures. When properly applied, this provides the resultant amplifier with characteristics of enhanced gain stability, greater bandwidth, lower distortion, and usefully modified stage input and output impedance s.
A block diagram of Black’s datasneet feedback amplifier system is shown in Figure H-1 below. Thus a few passive components, typically just resistors or sometimes reactive networks, set the gain and frequency response characteristics of this system.
Almost every op amp application ever conceived uses feedback.
But, a significant reason that Black’s feedback concept took root and prospered wasn’t simply because it was a useful and sound idea. That it was, but it was also different, and many experienced engineers fought the idea of “throwing away gain. By this help what is meant is that he had the full backing of the H.
The and years at Bell Labs could very well be regarded as golden years. They produced not just Black’s feedback amplifier, but also other key technical developments that expanded and supported datqsheet amplifier. This support came from some of the ad5477 finest engineers— not just the finest Bell Labs engineers, but the world’s finest.
Bode later published a classic feedback amplifier textbook see Reference Later on, he also gave a talk summarizing his views on the feedback amplifier’s development see Reference In addition to his famous stability criteria, Nyquist also supplied circuit-level hardware concepts, such as a patent on direct-coupled amplifier inter-stage coupling see Reference This idea was later to become a standard coupling method for vacuum tube op amps.
Outside Bell Labs, other engineers also were working on feedback amplifier applications of their own, dahasheet the concept in diverse practical applications. Frederick Terman was among the first to publicize the concept for AC feedback amplifiers, in a article see Reference Stewart Miller’s article offered techniques for high and stable gain with response to DC see Reference This article introduced what later became a standard gain stabilization concept, called “cathode compensation,” where a second dual triode section is used for desensitization of heater voltage variations.
Ginzton’s amplifier article employed Miller’s cathode compensation, as well as Nyquist’s level shifting method see Reference The level shifter is attributed to Brubaker who apparently duplicated Nyquist’s earlier work. Artzt’s article surveys various DC amplifier techniques, with emphasis on stability see Reference After World War II, the MIT Radiation Laboratory textbook series documented many valuable electronic techniques, including a volume dedicated to vacuum tube amplifiers.
The classic Valley-Wallman volume number 18 is not only generally devoted to amplifiers, it includes a chapter on DC amplifiers see Reference While this book doesn’t discuss op amps by name, it does include DC feedback circuitry examples. Op amps did exist, and had even been named as ofjust prior to the book’s publication. See also UK Patent 24, filed Nov. The ‘Fleming diode’ or the first vacuum tube rectifier.
Use of feedback to control amplifier output impedance. Another feedback amplifier development, generally paralleling Black’s. A simple one stage feedback amplifier system. A perspective discussion of the interrelated events and cultures surrounding the feedback amplifier’s invention. The prediction of feedback amplifier stability by means of semi-log gain-phase plots. Bode’s classic text on network analysis, as it relates to the design of feedback amplifiers.
A perspective historical summary of the author’s thoughts on the development of the feedback amplifier. A means of direct-coupling multiple amplifier stages via resistance networks for inter-stage coupling. Some practical AC-coupled topologies for implementing feedback amplifiers. Design example of a stable, high-gain direct-coupled amplifier including ‘cathodecompensation’ against variations in filament voltage, use of glow tube inter-stage coupling, and a stable line-operated DC supply.