Triode – Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Triode
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In the triode, electrons are released into the tube from the metal cathode by heating it, a process called thermionic emission. The cathode is heated red hot by a separate current flowing through a thin metal filament. In a few triodes, the filament itself is the cathode, while in most the filament heats a separate cathode electrode. Virtually all the air is removed from the tube, so the electrons can move freely. The negative electrons are attracted to the positively charged plate (anode), and flow through the spaces between the grid wires to it, creating a current through the tube from cathode to plate.
The magnitude of this current can be controlled by a voltage applied between the cathode and the grid. The grid acts like a gate for the electrons. A more negative voltage on the grid will repel some of the electrons, so fewer get through to the plate, reducing the plate current. A positive voltage on the grid will attract more electrons from the cathode, so more reach the plate, increasing the plate current. Therefore, a low power varying (AC) signal applied to the grid can control a much more powerful plate current, resulting in amplification. Variation in the grid voltage will cause identical proportional variations in the plate current. By placing a suitable load resistance in the plate circuit, the varying current will cause a varying voltage across the resistance which can be much larger than the input voltage variations, resulting in voltage gain.
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