Triodes
A third element-the control grid was added between the cathode and plate of a diode and so it provided the resulting triode tube with the ability to amplify tiny signals. The control grid in this tube is a circular spiral of a number of turns of fine wire that surround the cathode. Because of its open construction the grid does not directly hinder the flow of electrons to the plate, but when a voltage is placed on the grid it has a profound effect on the electron flow.
Since the control grid is nearer to the cathode than the plate, a potential placed on the grid has a much larger effect on the electric field within the tube- and hence upon the plate current-than the same potential placed on the plate. The grid thus has a controlling effect on the flow of plate current in the tube.
A triode requires three operating voltages, one on each electrode, to operate correctly. The plate of the tube is connected to a high positive voltage. E voltage a.c or d.c is connected the filament or heater to bring the cathode to its proper emitting temperature and thus make available a supply of electrons.
Finally, a voltage is placed on the control grid to govern the flow of plate current. This voltage generally consists of two components. One is a fixed dc. voltage, called the bias E which is normally a few volts negative with respect to the cathode. Its purpose is to operate the tube on a definite point on its characteristic curve so that a certain amount of plate current is always flowing.
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