Amplifiers
Amplifiers are not only a fundamental component of radios, televisions and telephones, they are essential to all modern electronics. Amplifiers differ considerably in design and in the amount of amplification (called gain) they produce, but they all work in much the same way.
The evolution of the amplifier began with Thomas Alva Edison who, in the process of studying his direct current light bulbs, inserted a metal plate near the filament. He discovered that electricity would flow from the positive side to the plate, diode, but treated it as curiosity.
John Ambrose Fleming, who worked for Edison, modified the diode in 1904 to detect radio waves. The Englishman called his invention the thermionic valve because it controlled the flow of water, but in the United States, it was called a vacuum tube, which better described its construction.
In 1906, American scientist Lee de Forest added a third element, called a grid, to Fleming's invention. De Forest discovered the device, which he called an "audion" (now known as a triode), made a superior radio wave detector. What he did not realize was his audion set up an electrical current that could be amplified considerably.
In 1912 Edwin Howard Armstrong discovered the amplifying capabilities of the audions by linking several of them together. He applied for a patent for his regenerative circuit and was sued by de Forest, who claimed the invention was his. The case dragged on for twenty years; the courts initially found in favor of Armstrong, but de Forest convinced the Supreme Court to ooverturn the findings.
Basically, the amplifier uses two circuits: a weak circuit in one part of the tube and a stronger current in another part. The flow of the weak current induces the flow of the strong current. When the weak current is modulated, it passes the modulation to the stronger current, which reproduces it at a much higher power. The stronger current is then sent into a loudspeaker where the signal is converted into sound waves.
Today transistors serve the same function as vacuum tubes. In a transistor the weak current flows between the emitter and the base, which induces a strong current to flow between the emitter and the collector. Because the emitter is shared by both the weak and strong current, transistors have only three terminals, as oppposed to four in a vacuum tube.
Triodes have a major flaw; they amplify distortion as well as audio. In 1923 Harold S. Black discovered that he could subtract the amplitude of the output signal from that of the input signal and cancel both signals. That left just the distortion which could be amplified, fed back into the system, and used to cancel out the original distortion. This "feedback-feedforward" system did not completely eliminate distortion, but it reduced it considerably. Three years later he discovered that by taking the the output signal of the amplifier and feeding it back into the system out of phase (negative feedback), he could obtain nearly any amount of distortion reduction.
Amplifiers today are used in devices never imagined by the early inventors: compact disk players, tape recorders, radar, digital computers, servomechanisms, and electronic musical instruments are only a few of the inventions that depend upon amplifiers for their operation.
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