Harold S. Black, negative feedback amplifier

Harold S. Black, Bell Labs

Black (courtesy AT&T)

In the first part of the twentieth century, long-distance calls were noisy at best. Transcontinental voice calls were not common at all. Why? Because the farther down the line a caller's intended receiver was, the more often the voice signal needed to be amplified, and the more distortion of the voice signal occurred due to increasing resistance.

AT&T's (NYSE: T) newly developed three-channel Type C telephone system used repeater amplifiers to improve signaling, but distortion was a big problem. So the company's Systems Engineering Dept. (Bell Labs) put electrical engineer Harold S. Black on the problem in 1925. The Worcester Polytechnic Institute (WPI) graduate developed a feed-forward amplifier, which was able to cancel out some of the distortion in a long distance call, but required so much adjustment in current and voltage that it was not introduced commercially.

In August 1927, Black was taking the Lackawanna Ferry across the Hudson River on his way to work when a new idea occurred to him. "He immediately sketched a block diagram and equations for an amplifier with feedback on a blank page of his newspaper," an IEEE biography noted.

The idea was not to go forward, but backward: "Black discovered that if a portion of an amplifier's output were fed back into the amplifier in negative phase, the inverted distortions would cancel out the distortions introduced by the amplifier." (Michael W. Dorsey, WPI Transformations)

By late 1927, Black's prototype negative feedback amplifier "achieved a distortion reduction of 100,000 to 1 with a frequency range extending from 4 to 45 kHz." AT&T began testing the negative feedback amplifier in a nine-channel carrier system it was developing for transcontinental voice calling.

The performance of Black's negative feedback amplifiers received a significant boost with the contribution of colleague Harry Nyquist, whose work on stabilizing feedback circuits played an essential role in their development.

The negative feedback principle itself has applications across technological and educational fields as a mechanism for maintaining equilibrium in various systems.

Describing the nuts and bolts of telephony can be dry stuff, but to illustrate how mind-blowing Black's work was, consider that the U.S. Patent and Trademark Office, which readily approved his feed-forward amplifier patent application in 1928, took some nine years to approve his patent for the negative feedback amplifier. "He attributed the unusual delay to the fact ‘that the concept was so contrary to established beliefs that the Patent Office initially did not believe it would work.'" (Proceedings of the IEEE, Vol. 99, No. 2, Feb. 2011)