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along the way. Every time someone spoke, there was a rise and fall in background static, so bad it was described as “like a distant train whistle over a stormy sea.” The repeater amplifier problem was considered unsolvable to everyone because nobody knew how to make the amplifier linear or stable enough. For years, the only
near a guitar amplifier. Quickly, the signal will ‘self-oscillate’ and it will start wailing a high-frequency ear- bursting whine. This skepticism of the concept would follow him for the next several years.
Retro Electro fun fact: while the reader can find ‘AA,’ ‘C,’ and ‘D’ batteries at the local hardware store, ‘A’ and ‘B’ batteries are absent. ‘B’ batteries were used in early vacuum tube equipment to provide a plate voltage of up to ninety volts, while an ‘A’ battery was used to supply high current to heat the filaments in the tubes.
Retro Electro fun fact: this is the underlying concept that makes modern operational amplifiers (Op-Amps) work.
of the ‘B’ battery. Ultimately, these solutions were too complicated and complex to be practical.
approach to a solution was to try to make a perfect ‘linear’ vacuum tube. All this research later proved that it is impossible.
First principles After working on this problem for over a year, Black was plainly stumped. The unsolvable problem. Then, in March of 1923, Black attended a lecture by mathematician and electrical engineer Charles Steinmetz. Black arrived early to get a seat in the front row. Steinmetz arrived 20 minutes late, walking down the aisle with enormous applause and an even larger cigar hanging out of his mouth. Black recounts this lecture by saying that “ I no longer remember the subject, but I do remember the clarity and logic of his presentation and how quickly and directly he reached the final conclusion of his talk.” Black was so impressed by this approach that he went home and restated his amplifier in a new way. Removing all of the superfluous requirements of gain and distortion rejection. He framed his new problem simply as “remove all distortion products from the amplifier output.” The amplifier output consisted of two different parts: the wanted signal or intelligence and the unwanted distortion. Anything that was not part of the desired signal was now considered distortion. Now
Within a few weeks. He had a working model that gave a
reduction in distortion of 100,000 to 1 in a single amplifier module, finally solving the task he had been assigned six years earlier. These results were quickly signed and sent off to Bell Labs’ patent lawyer, Harry A. Burgess. In January 1928, Bell Labs began developing a new system for transcontinental cables. This became the first application of the invention. Each amplifier was The Negative Feedback Amplifier works by feeding the output through a ‘feedback circuit’ back into the input of the amplifier.
In a flash Then, after four years of trial and error, while riding the ferry to the city from his home in New Jersey, it came to him ‘in a flash.’ He realized that if he fed the amplifier output back into the input, but 180 degrees out of phase, he would have what he had been wanting. “A means to cancel out the distortion in the output.” He took what paper he had with him, that day’s copy of the New York Times, and scrawled out the diagrams and formulas needed for the negative feedback amplifier. As soon as the ferry stopped, he ran to the West
The newspaper that Black wrote out the solution he received ‘in a flash’
to “isolate and then eliminate this distortion.” He now considered that if he reduced an amplified output signal from an amplifier to the same amplitude as the input signal and then subtracted the input signal from the adjusted output, the only thing that would remain would be the distortion from the amplifier. He could take this distortion, using a separate amplifier, amplify it, then subtract it from the original amplifier output, thereby relieving the system of the distortion. Surely this felt like the perfect solution at the time, but it was still far from it. He designed a ‘feed-forward amplifier’ with this solution, but every hour, twenty-four hours a day, a technician had to make adjustments to the vacuum tube’s current. Then, four times a day, someone had to adjust the voltages
Street Labs (by then renamed to Bell Labs) and had it witnessed by Earl C. Blessing. Few people had faith that it could work. Feeding the output of an amplifier back into the input just sounds like loud noise. Consider what happens when a microphone is placed
Charles Steinmetz (Right) and Albert Einstein (Left) in 1921
we get technical
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