Harold Stephen Black

Harold Stephen Black

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Harold Stephen Black was an American electrical engineer, who revolutionized the field of applied electronics by inventing the negative feedback amplifier in 1927. To some, his invention is considered the most important breakthrough of the twentieth century in the field of electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

, since it has a wide area of application. This is because all electronic devices (vacuum tubes, bipolar transistors and MOS transistors) invented by mankind are basically nonlinear devices. It is the invention of negative feedback which makes highly linear amplifiers possible. Negative feedback basically works by sacrificing gain for higher linearity (or in other words, smaller distortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...

 or smaller intermodulation
Intermodulation or intermodulation distortion is the amplitude modulation of signals containing two or more different frequencies in a system with nonlinearities...

). By sacrificing gain, it also has an additional effect of increasing the bandwidth of the amplifier. However, a negative feedback amplifier can be unstable such that it may oscillate. Once the stability problem is solved, the negative feedback amplifier is extremely useful in the field of electronics. Black published a famous paper, Stabilized feedback amplifiers, in 1934.


He was born in Leominster, Massachusetts
Leominster, Massachusetts
Leominster is a city in Worcester County, Massachusetts, United States. It is the second-largest city in Worcester County, with a population of 40,759 at the 2010 census. Leominster is located north of Worcester and west of Boston. Both Route 2 and Route 12 pass through Leominster. Interstate 190,...

 in 1898. He went to Worcester Polytechnic Institute
Worcester Polytechnic Institute
Worcester Polytechnic Institute is a private university located in Worcester, Massachusetts, in the United States.Founded in 1865 in Worcester, WPI was one of the United States' first engineering and technology universities...

 (WPI) for his first degree. Subsequently, he received the B.S.E.E. degree from WPI in 1921 and then joined Western Electric
Western Electric
Western Electric Company was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. It was the scene of a number of technological innovations and also some seminal developments in industrial management...

, which was the manufacturing arm of AT&T. He joined Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...

 (1925), where he was a member of technical staff until his retirement (1963).

Black started writing his autobiography with the tentative title "Before the ferry docked". However, he died in December 1983 at age 85 before he could finish it.


Amplifiers are non-linear, therefore every time a signal is amplified in a telecommunications network, which can happen dozens of times on a circuit, noise and distortion are added. Black first invented the feed-forward amplifier which compares the input and output signals and then negatively amplifies the distortion and combines the two signals, canceling out some of the distortion. This amplifier design improved, but did not solve, the problems of transcontinental telecommunication.

After years of work Black invented the negative feedback amplifier which uses negative feedback to reduce the gain of a high-gain, non-linear amplifier and makes it act as a low-gain, linear amplifier with much lower noise and distortion. The Negative feedback amplifier allowed Bell system to reduce overcrowding of lines and extend its long-distance network by means of carrier telephony. It enabled the design of accurate fire-control systems in World War II, and it formed the basis of early operational amplifiers, as well as precise, variable-frequency audio oscillators.

According to Black he got his inspiration to invent the negative feedback amplifier when he was traveling from New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...

 to New York City
New York City
New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...

 by taking a ferry to cross the Hudson River
Hudson River
The Hudson is a river that flows from north to south through eastern New York. The highest official source is at Lake Tear of the Clouds, on the slopes of Mount Marcy in the Adirondack Mountains. The river itself officially begins in Henderson Lake in Newcomb, New York...

 in August 1927. Having nothing to write on he sketched his thoughts on a misprinted page of the New York Times and then signed and dated it. At that time, Bell Laboratories headquarters were located in 463 West Street, Manhattan
Manhattan is the oldest and the most densely populated of the five boroughs of New York City. Located primarily on the island of Manhattan at the mouth of the Hudson River, the boundaries of the borough are identical to those of New York County, an original county of the state of New York...

, New York City
New York City
New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...

 instead of New Jersey and he lived in New Jersey such that he took the ferry every morning to go to work.

Fifty years after his 1927 invention, he published an article in IEEE Spectrum regarding the historical background of his invention. He published a classical paper on negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

 amplifier in 1934, which has been re-printed in the Proceedings of IEEE two times in 1984 and 1999 Inside his 1934 classical paper "Stabilized feed-back amplifiers", he mentioned Harry Nyquist
Harry Nyquist
Harry Nyquist was an important contributor to information theory.-Personal life:...

's work on stability criterion because a negative feedback amplifier can be unstable and oscillate. Thus, with the help of Nyquist's theory, he managed to demonstrate a stable negative feedback amplifier which can be used in reality. Bernard Friedland wrote an introduction for the 1999 re-print in Proc. IEEE. James E. Brittain wrote about him in 1997.
An obituary regarding Black was published by IEEE Transactions on Automatic Control in 1984.

He also worked on pulse code modulation and wrote a book on "Modulation Theory" (Van Nostrand, 1953). He held many patents the most famous of which was US Patent 2,102,671 "Wave Translation System", which was issued to Bell Laboratories in 1937, covering the negative feedback amplifier.


  • National Inventors Hall of Fame
    National Inventors Hall of Fame
    The National Inventors Hall of Fame is a not-for-profit organization dedicated to recognizing, honoring and encouraging invention and creativity through the administration of its programs. The Hall of Fame honors the men and women responsible for the great technological advances that make human,...

     inductee number 25, 1981
  • Robert H. Goddard
    Robert H. Goddard
    Robert Hutchings Goddard was an American professor, physicist and inventor who is credited with creating and building the world's first liquid-fueled rocket, which he successfully launched on March 16, 1926...

     Award from WPI 1981
  • AIEE Lamme Medal 1958
  • D. Eng. degree (honorary) from WPI 1955
  • Research Corporation
    Research Corporation
    The Research Corporation for Science Advancement is an organization in the United States devoted to the advancement of science, funding research projects in the physical sciences. It was also a major supporter of the research that led to the presentation of Interlingua in 1951...

     Scientific Award 1952
  • John H. Potts Memorial Award of the Audio Engineering Society
    Audio Engineering Society
    Established in 1948, the Audio Engineering Society draws its membership from amongst engineers, scientists, other individuals with an interest or involvement in the professional audio industry. The membership largely comprises engineers developing devices or products for audio, and persons working...

  • John Price Wetherill Award of the Franklin Institute
    Franklin Institute
    The Franklin Institute is a museum in Philadelphia, Pennsylvania, and one of the oldest centers of science education and development in the United States, dating to 1824. The Institute also houses the Benjamin Franklin National Memorial.-History:On February 5, 1824, Samuel Vaughn Merrick and...

  • Certificate of Appreciation from the US War Department.
  • WPI gives away the annual Harold S. Black Scholarship (1992-)