Harold Stephen Black (April 14, 1898 – December 11, 1983) 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, since it has a wide area of application.

Once the stability problem is solved, the negative feedback amplifier is extremely useful in the field of electronics.

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.

[2] According to Black[3] he got his inspiration to invent the negative feedback amplifier when he was traveling from New Jersey to New York City by taking a ferry to cross the Hudson River in August 1927.

Thus, with the help of Nyquist's theory, he managed to demonstrate a stable negative feedback amplifier which can be used in reality.