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Op Amp and Analog Computing Developments

Some of the differential amplifier work described above did find its way into op amps. But, there was also much other significant amplifier work being done, at Bell Labs and elsewhere in the US, as well around the world. The narrative of op amp development now focuses on the thread of analog computing, which was the first op amp application.
In the late 1930’s George A. Philbrick, at Foxboro Corporation, was developing analog process control simulation circuits with vacuum tubes and passive parts. Philbrick developed many interesting circuits, and some were op amp forebears (Reference 21: George A. Philbrick, "Designing Industrial Controllers by Analog," Electronics, June 1948. (Fig. 3b example of an early analog computation circuit)).
In fact, within this article he describes a single tube circuit that performs some op amp functions, in Figure 3A. This directly coupled circuit develops an operating relationship between input and output voltages, producing a voltage output proportional to the ratio of two impedances. While this circuit (using floating batteries for power) can’t be termed a general-purpose op amp circuit, it nevertheless demonstrated some of the working principles. In Reference 22(Per A. Holst, "George A. Philbrick and Polyphemus–— The First Electronic Training Simulator," IEEE Annals of the History of Computing, Vol. 4, No. 2, April 1982, pp. 143-156. (George A. Philbrick’s analog computing circuitry at Foxboro, late 1930’s)) Per Holst further describes this early Philbrick work. Within a decade Philbrick was to start his own company supplying vacuum tube op amps and other components used within analog simulation schemes.
But, the very first vacuum tube amplifiers fitting the introductory section op amp definition came about early in the 1940’s wartime period. The overall context was the use of this amplifier as a building block, within the Bell Labs designed M9 gun director system used by WWII Allied Forces. These op amp circuits were general purpose, using bipolar supply voltages for power, handling bipolar input/output signals with respect to a common voltage (ground). As true to the definition, the overall transfer function was defined by the externally connected input and feedback impedances.
These early amplifiers were part of a specialized analog computer system, which was designed to calculate proper gun aiming for fire upon enemy targets. The work on this project started in 1940, and was pioneered by Clarence A. Lovell, David Parkinson, and many other engineers of the Bell Labs staff. Their efforts have been chronicled in great detail by Higgins, et al, as well by James S. Small (References 23 and 24: H. C. Higgins, et al, "Defense Research at Bell Laboratories: Electrical Computers for Fire Control," IEEE Annals of the History of Computing, Vol. 4, No. 3, July, 1982, pp. 218-236. See also: M. D. Fagen, Ed. A History of Engineering and Science in the Bell System, Vol. 2, "National Service in War and Peace, 1925-1975," Bell Telephone Laboratories, 1978, ISBN 0-0932764-001-2. (A summary of various Bell Labs fire control analog computer developments, including op amps and feedback components. Note— the IEEE article is reprinted from the broader, more detailed Bell Telephone Laboratories volume) and James S. Small, "General-Purpose Electronic Analog Computing: 1945-1965," IEEE Annals of the History of Computing, Vol. 15, No. 2, 1993, pp. 8-18. (An overview of analog computing, includes discussion of early operational amplifiers)).
This Bell Labs design project resulted in a prototype gun director system that was called the T10, first tested in December of 1941. While the T10 was the first sample gun director, in later production the gun director was known as the Western Electric M9 (Reference 25: David A. Mindell, "Automation’s Finest Hour: Bell Labs and Automatic Control in World War II," IEEE Control Systems, December 1995, pp. 72-80. (Narrative of the T10 computer system and the M9 gun director developments at Bell Labs)). Further documentation of this work is found in US Patents 2,404,081 and 2,404,387 (References 26 and 27: C. A. Lovell, et al, "Artillery Predictor," US Patent 2,404,081, filed May 1, 1941, issued September 24, 1946. (The mathematics of analog computer system using op amps for functions of repeating, inverting and summing amplifiers, plus differentiation) and C. A. Lovell, et al, "Electrical Computing System," US Patent 2,404,387, filed May 1, 1941, issued July 23, 1946. (An analog computer system using op amps for control)), plus a related paper by Lovell (Reference 28: C. A. Lovell, "Continuous Electrical Computation," Bell Laboratories Record, 25, March, 1947, pp. 114-118. (An overview of various fire-control analog computational circuits of the T10 and M9 systems, many illustrating uses of op amps)). The patents illustrate many common feedback amplifier examples in varied tasks.
However, in terms of an overall technical view of the M9, perhaps the most definitive discussion can be found within "Artillery Director," US Patent 2,493,183, by William Boghosian, Sidney Darlington, and Henry Och of Bell Labs (Reference 29: W. H. Boghosian, et al, "Artillery Director," US Patent 2,493,183, filed May 21, 1942, issued Jan. 3, 1950. (An artillery fire control system using op amps for control)). This key document breaks down the design of the analog computation scheme into the numerous subsystems involved. Op amps can be found throughout the patent figures, performing functions of buffers, summers, differentiators, inverters, etc.
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