Negative Feedback Theory

Invented by Harold Black, Class of 1921

Black was a young engineer at Bell Laboratories in New York City in 1927 when he invented the negative-feedback amplifier, a discovery that played a vital role in 20th-century electronics. It eliminated distortion from telephone calls, was used in gun-control systems during the Second World War, and was key to a range of postwar electronics--from computers to pacemakers to high-fidelity recordings. It is still widely used in control and communications systems today. In 1957, Mervin Kelly, then president of Bell Labs, called the achievement one of two inventions that had the broadest impact on electronics and communications during the previous half century.

Black's invention solved a problem that had hindered the advance of long-distance telephone service. To get telephone signals to travel over long spans, one had to amplify them several times, with each amplification introducing new distortions. Black's insight was that by feeding part of the signal back into the amplifier, in negative phase, and comparing it to the original signal, the distortion could be greatly reduced.

Black's technical accomplishment was important enough to earn him eternal recognition (as just one example of the honors he received, he was inducted into the Inventors Hall of Fame, along with Robert Goddard '08). But the story of his discovery is also the stuff of legend. In 1927 he was taking the Hudson River ferry to his office when he suddenly thought of the solution to the distortion problem he'd been absorbed in for some weeks. Having nothing else to write on, he sketched his idea on a page of The New York Times that had been printed quite faintly. He signed and dated his notes and then had a colleague do the same when he got to work. That page from the Times, pictured above, is now in the AT&T archives.

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