Home » , , » Setting the Stage for the Op Amp

Setting the Stage for the Op Amp

Op amps are high gain amplifiers, and are used almost invariably with overall loop feedback. The principle of the feedback amplifier has to rank as one of the more notable developments 20th century— right up there with the automobile or airplane for 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. This is a clear tribute to the concept’s fundamental value.
Today the application of negative feedback is so common that it is often taken for granted. But this wasn't always the case. Working as a young Western Electric Company engineer on telephone channel amplifiers, Harold S. 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 after the broadly written 1928 patent application, until the 1937 issuance (Reference 3: H. S. Black, "Wave Translation System," US Patent 2,102,671, filed August 8, 1928, issued December 21, 1937 (The basis of feedback amplifier systems)). 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 (References 4 and 5: H. S. Black, "Stabilized Feedback Amplifiers," Bell System Technical Journal, Vol. 13, No. 1, January 1934, pp. 1-18 (A practical summary of feedback amplifier systems) and Harold S. Black, "Inventing the Negative Feedback Amplifier," IEEE Spectrum, December, 1977 (Inventor’s 50th anniversary story on the invention of the feedback amplifier) ).
But, like circumstances surrounding other key inventions, there were others working on negative feedback amplifier applications. One example would be Paul Voigt's mid-1920's work (References 6 and 7: Geoffrey Horn, "Voigt, not Black," Stereophile, Letters, April 1998, pp. 18, 21 and Paul G. A. H. Voigt, "Improvements in or Relating to Thermionic Amplifying Circuits for Telephony," UK Patent 231,972, filed January 29, 1924, issued April 16, 1925. (A motional feedback loudspeaker/amplifier system) )(Some suggest Paul Voigt as the true feedback amplifier inventor, not Black (see Ref. 6, 7). Examination of Voigt's UK patent 231,972 fails to show a feedback amplifier theory comparable to Black's detailed exposition of Ref. 3 and 4. In fact, there are no equations presented to describe Voigt's system behavior). The prolific British inventor Alan Blumlein did 1930's feedback amplifier work, using it to control amplifier output impedance (Reference 8: A. D. Blumlein, "Improvements in and relating to Thermionic Valve Amplifiers," UK Patent 425,553, filed Sept. 8, 1933, issued March 18, 1935. (Use of feedback to control amplifier output impedance))(Blumlein's UK patent 425,553 is focused on controlling amplifier output impedance through voltage and/or current feedback, not addressing in detail the broader ramifications of feedback). Finally, a research group at N. V. 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). In 1937 B. D. H. Tellegen published a paper on feedback amplifiers, with attributions to K. Posthumus and Black (References 9 and 10: B. D. H. Tellegen, "Inverse Feedback," Philips Technical Review, Vol. 2, No. 10, October, 1937. (Another feedback amplifier development, generally paralleling Black's) and  "Improvements in or Relating to Arrangements for Amplifying Electrical Oscillations," UK Patent 323,823, filed October 18, 1928, issued January 16, 1930 (original filing), filed July 18, 1929, final approval January 1938 and February 1939 (amended filing). (A simple one stage feedback amplifier system))(Examination of UK patent 323,823 fails to find reference to K. Posthumus, apparently a practice with N. V. Philips UK patents of that period. The patent does show a rudimentary feedback amplifier, but unfortunately the overall clarity is marred by various revisions and corrections, to both text and figures). In Tellegen's paper are the same equations as those within Black's (substituting A for Black's μ).
But, it isn't the purpose here to challenge Black's work, rather to note that 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 a most significant effort. It is also both broad-based and widely accepted.
Black's Feedback Amplifier
The basis of Black’s feedback amplifier lies in the application of a portion of the output back to the input, so as to reduce the overall gain. 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 basic feedback amplifier system is shown in Figure 1 below. Note that Black’s "μ" for a forward gain symbol is today typically replaced by "A". As so used, the feedback network β defines the overall transfer expression of the amplifier. Thus a few passive components, typically just resistors or sometimes reactive networks, set the gain and frequency response characteristics of this system.
A block diagram of Black’s feedback amplifier, comprised of a forward gain "μ" and a feedback path of "β".
Figure 1: A block diagram of Black’s feedback amplifier, comprised of a forward gain "μ" and a feedback path of "β".
At the time Black’s work was initiated, the problem he faced was how to make practical a series signal connection of hundreds (if not thousands) of telephone system repeater amplifiers using directly heated triode tubes. The magnitude of this problem becomes obvious when it is considered that each amplifier alone couldn’t be held more stable to less than 1dB of gain variation, and even under the best of conditions, the stage distortion was unacceptable.
Black’s feedback amplifier invention led not just to better repeater amplifiers for Western Electric, but to countless millions of other widely varying applications. Almost every op amp application ever conceived uses feedback. So, given the fact that modern op amp types number in the dozens (in individual models, many thousands), it isn’t hard to appreciate the importance feedback principles take on for today’s designs.
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." However, Black did have help in selling the radically new concept, help that was available to few other inventors. By this help what is meant is that he had the full backing of the Bell Telephone System, and all that this implied towards forging and promoting a new technical concept. An interesting narrative of the feedback amplifier's development and the interplay of Black and his coworkers can be found in David Mindell's paper, "Opening Black’s Box: Rethinking Feedback’s Myth of Origin" (Reference 12: David A. Mindell, "Opening Black’s Box: Rethinking Feedback’s Myth of Origin," Technology and Culture, Vol. 41, July, 2000, pp. 405-434. (A perspective discussion of the interrelated events and cultures surrounding the feedback amplifier's invention) ).
The 1930 and 1940 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 the amplifier. This support came from some of the period's finest engineers— not just the finest Bell Labs engineers, but the world's finest.
To quote Black’s own words on the Bell Labs support activity related to his landmark invention, "Within a few years, Harry Nyquist would publish his generalized rule for avoiding instability in a feedback amplifier, and Hendrick W. Bode would spearhead the development of systematic techniques of design whereby one could get the most out of a specified situation and still satisfy Nyquist’s criterion." (see Reference 5, again).
The feedback amplifier papers and patents of Harry Nyquist and Hendrick Bode (References 13 and 14: Harry Nyquist, "Regeneration Theory," Bell System Technical Journal, Vol. 11, No. 3, July, 1932, pp. 126-147. See also: "Regenerative Amplifier," US Patent 1,915,440, filed May 1, 1930, issued June 27, 1933 (The prediction of feedback amplifier stability by means of circular gain-phase plots) and Hendrick Bode, "Relations Between Attenuation and Phase In Feedback Amplifier Design," Bell System Technical Journal, Vol. 19, No. 3, July, 1940. See also: "Amplifier," US Patent 2,123,178, filed June 22, 1937, issued July 12, 1938. (The prediction of feedback amplifier stability by means of semi-log gain-phase plots) ), taken along with the body of Black’s original work, form solid foundations for modern feedback amplifier design. Bode later published a classic feedback amplifier textbook (Reference 15: Hendrick Bode, Network Analysis and Feedback Amplifier Design, Van Nostrand, 1945. (Bode's classic text on network analysis, as it relates to the design of feedback amplifiers) ). Later on, he also gave a talk summarizing his views on the feedback amplifier's development (Reference 16: Hendrick Bode, "Feedback— the History of an Idea," Proceedings of the Symposium on Active Networks and Feedback Systems, Polytechnic Press, 1960. Reprinted within Selected Papers on Mathematical Trends in Control Theory, Dover Books, 1964. (A perspective historical summary of the author's thoughts on the development of the feedback amplifier) ).

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 (Reference 17: Harry Nyquist, "Distortionless Amplifying System," US Patent 1,751,527, filed November 24, 1926, issued March 25, 1930. (A means of direct-coupling multiple amplifier stages via resistance networks for inter-stage coupling) ). 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, affirming the concept in diverse practical applications. Frederick Terman was among the first to publicize the concept for AC feedback amplifiers, in a 1938 article (Reference 18: F. E. Terman, "Feedback Amplifier Design," Electronics, January 1937, pp. 12-15, 50. (Some practical AC-coupled topologies for implementing feedback amplifiers) ).
For single-ended signal path DC amplifiers, there were numerous landmark papers during the World War II period. Stewart Miller's 1941 article offered techniques for high and stable gain with response to DC (Reference 19: Stewart E. Miller, "Sensitive DC Amplifier with AC Operation," Electronics, November, 1941, pp. 27-31, 106-109. (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)). 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 1944 amplifier article employed Miller's cathode compensation, as well as Nyquist's level shifting method (Reference 20: Edward L. Ginzton, "DC Amplifier Design Techniques," Electronics, March 1944, pp. 98-102 (Various design means for improving direct-coupled amplifiers)). The level shifter is attributed to Brubaker (who apparently duplicated Nyquist's earlier work). Artzt's 1945 article surveys various DC amplifier techniques, with emphasis on stability (Reference 21: Maurice Artzt, "Survey of DC Amplifiers," Electronics, August, 1945, pp. 112-118. (Survey of directcoupled amplifier designs, both single-ended and differential, with emphasis on high stability)).
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 (Reference 22: George E. Valley, Jr., Henry Wallman, Vacuum Tube Amplifiers, MIT Radiation Labs Series No. 18, McGraw-Hill, 1948. (A classic WWII Radiation Lab development team textbook. Chapter 11, by John W. Gray, deals with direct-coupled amplifiers)). 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 of 1947, just prior to the book's publication.
Share this article :


Post a Comment

Please wait for approval of your comment .......