Submitted by Marc Bell, Editor
Copyright 1997 IEEE. Reprinted with permission from the IEEE publication, “
Harold S. Black and the Negative Feedback Amplifier
Scanning the Past” which covers a reprint of an article appearing in the Proceedings of the IEEE Vol. 85, No. 8, August 1997.
Seventy years ago this month, H. S. Black (Fig. 1) of Bell Telephone Laboratories conceived the negative feedback amplifier while aboard the Lackawanna Ferry on his way to work. Thirty years later, M. J. Kelly, president of the Bell Labs, characterized Black’s invention as ranking with the de Forest audion “as one of the two inventions of broadest scope and significance in electronics and communications of the past 50 years.” Kelly credited the negative feedback amplifier with having made possible the long-distance telecommunications networks that covered the country, as well as transoceanic telephone cables. He noted that by 1957, the application of the negative feedback principle had transcended telecommunications and had stimulated “the entire explosive extension of the area of control, both electrical and mechanical.”
Black was born in Leominister, MA, in 1898 and graduated in electrical engineering from the Worcester Polytechnic Institute in 1921. That year, he joined the Systems Engineering Department of the Western Electric Company in New York City, which became part of Bell Laboratories in 1925. He frequently came to the office on Sundays to peruse technical reports on projects covering the past two decades. His initial assignment was to investigate distortion in vacuum-tube amplifiers used as repeaters in telephonic carrier systems. He undertook a laborious analysis of distortion and linearity requirements as a function of the number of channels and designed various amplifiers in a quest for circuits suitable for multichannel amplifiers used in tandem over long distances. During the 1920’s, he worked closely with Kelly, who was in charge of vacuum-tube research.
In March 1923, Black attended an inspiring talk by C. P. Steinmetz, which, according to Black, provided a stimulus to his invention of a “feedforward amplifier.” This invention, which he patented in 1928, utilized biconjugate networks to isolate and cancel distortion. The technique proved fairly successful in laboratory tests but required frequent adjustment of the filament current and plate voltage Continue reading