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115th Birthday of Harold S. Black, Inventor of the Negative Feedback Amplifier

Today would be the 115th birthday of Harold Stephen Black, who invented the negative-feedback amplifier in 1927. The story goes that Mr. Black was taking the ferry from New Jersey to New York City (where Bell Labs was then located), and while on the boat he was struck by the idea to use negative feedback to linearize an amplifier stage.

The idea of negative feedback is a powerful one, because it provides a means to construct amplifiers with precise gain characteristics from parts with less than ideal specs. This makes it easier to build circuits with interchangeable, off-the-shelf parts. Gain can be adjusted using passive components, which are cheaper and easier to adjust.

Distortion and intermodulation are reduced significantly, and bandwidth is greatly increased. It also enables the construction of higher order filters without the use of inductors, which are heavy and prone to other problems. Though initially developed for vacuum tubes, the idea is very useful in transistor circuits, which can have wildly varying beta characteristics, even among devices on the same chip.

There are a lot of things to like about the video above, but my favorite part is the way he talks so earnestly about his failures, before finally succeeding.

Happy Birthday, Mr. Black!

Further reading:

Black’s Bell System Technical Journal article, Stabilized Feed-back Amplifiers, 1934 (PDF)

Harold Black’s IEEE Hall of Fame profile (PDF)

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  1. Thanks for the post. Haven’t thought about negative feedback and bandwidth for years.

  2. Is he the one who added a wire from the Op Amp output to its negative input?

  3. @miros: no, he’s the guy who _invented_ the negative input.

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