retroelectro
amplifier back into its input and achieve a gain of more than one. The concept of negative feedback was totally unknown. The patent examiners would cite numerous technical journals showing that it’s impossible that it could work. In England, Black says in an article, his “patent application was treated in the same manner as one for a perpetual motion machine.” The patent was long. It had 75 different figures and diagrams, nine pages of 126 different claims, and 42 pages of text. Since the concept of negative feedback was brand new, much of the patent was spent trying to teach the concept, making it difficult and time-consuming to go through. Eventually, Bell Labs was able to overcome these problems by demonstrating that 70 amplifiers were currently functioning successfully in the telephone building at Morristown, New Jersey. Bell Labs v Zenith The issue with patents is that they are only as good as the holder’s ability to defend them. Bell Laboratories aggressively enforced the negative-feedback amplifier patent against Zenith Radio Corp. beginning in 1948. Bell Labs alleged that Zenith’s FM and AM receiver designs incorporated feedback loops that infringed Black’s patent. Black was deposed and testified extensively over multiple sessions between 1948 and 1953, explaining
After more than four decades at Bell Labs, Black joined General Precision Corporation in 1963 as a Principal Research Scientist. Beginning in 1966, he transitioned to work as an independent communications consultant. By that time, Dr. Black had amassed 62 U.S. patents and 271 additional
patents across 32 countries. In addition to numerous technical papers, he authored the seminal text Modulation Theory in 1953. In recognition of his contributions, Worcester Polytechnic Institute awarded him an honorary Doctor of Engineering degree in 1955.
Suggested reading 1. The Bell Telephone: Patent Nonsense? (Washington Post, Feb 20, 2008) https://www.washingtonpost.com/wp-dyn/content/article/2008/02/19/ AR2008021902596.html?sub=AR 2. The ‘Musical Telegraph’ or ‘Electro-Harmonic Telegraph’, Elisha Gray. USA, 1874 https://120years.net/the-musical-telegraphelisha-greyusa1876/ 3. The Telephone Gambit: Chasing Alexander Graham Bell’s Secret by Seth Shulman 4. Harold S. Black, an oral history conducted in 1977 by Michael Wolff, IEEE History Center, Piscataway, NJ, USA. https://ethw.org/Oral-History:Harold_S._Black 5. Harold Black and the Negative Feedback Amplifier by Robert Kline https://brewer.ece.gatech.edu/ece3043/FBBlack.pdf 6. Inventing the Negative Feedback Amplifier – IEEE Xplore (December 1977) https://wiki.epfl.ch/me412-emem-2020/documents/06501721.pdf 7. Stabilized Feedback Amplifiers – The Bell System Technical Journal https://convexoptimization.com/TOOLS/Black.pdf 8. Harold S Black Patents (Google Patents) https://patents.google.com/?inventor=Harold+S+Black&sort=old https://patentimages.storage.googleapis.com/b8/63/f0/ e6bfd23228c3e1/US2102671.pdf 9. Modulation Theory by Harold S. Black https://babel.hathitrust.org/cgi/pt?id=mdp.39015002077488&seq=9
expected to transmit up to nine different voice channels over a single pair of 16-gauge wire in a twenty-five-mile-long underground cable between repeater stations. Once word of this started spreading at Bell Labs, the higher-ups at the company objected, claiming it was impossible for it to actually work. Instead, Black was instructed to design more traditional Colpitts push-pull amplifiers that could meet the requirements, and he did. Rarely discouraged, behind the scenes, he started designing six different models of his negative feedback amplifier, in addition to the other work. When he was able to demonstrate that these new amplifiers could easily meet the requirements of the proposed system, the management was satisfied, and he didn’t hear anything else about using the Colpitts amplifiers.
A couple of years later, his new system was ready for a field test. They built seventy-eight new negative feedback amplifiers and took them to the labs at Morristown, NJ. Here, they used a twenty-five-mile-long stretch of cable, made up of sixty-eight pairs, each expected to carry nine voice channels. With this cable, looping back on itself with every pair, they were able to simulate communication over a 7,650- mile distance, which resulted in excellent voice quality. Patent troubles Even though the patent application was sent off to the US Patent Office in 1928, it would be over nine years before the patent was issued. The main reason for this is that it was already well established that you cannot tie the output of an
Harold Black in 1981
both the genesis of his inverted- feedback concept and the technical distinctions between his licensed implementations and Zenith’s circuits. The litigation ultimately led Zenith to negotiate a licensing agreement with Bell Labs, under which Zenith paid royalties for use of the negative-feedback technique in its high-fidelity receivers. Legacy Nearly one hundred years later, the negative feedback amplifier concepts can be found in everything. It is used in Op-Amps, EVs, space satellites, cell phones, guitar amps, cruise missiles, prosthetic limbs, and in thousands of other applications. In this way, Harold S. Black is an inventor on the tier of Lord Kelvin, Hans Camenzind, and Philo Farnsworth.
The issue with patents is that they are only as good as the holder’s ability to defend them
we get technical
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