CHAPTER X

VACUUM TUBES AND VACUUM TUBE AMPLIFIERS

(A) VACUUM TUBES AND VACUUM TUBE CIRCUITS

119. The Vacuum Tube As an Amplifier.-The use of the vacuum tube as a rectifier was described in section 56. It will be remembered that the tubes discussed had but two elements, or electrodes, and that the current through the tube flowed in one direction only. The amplifier tube has three electrodes. The third electrode, called the grid, is used to control the flow of electrons through the tube. The action of this tube can best be described by going back to that which has already been discussed, namely, the emission of electrons from a heated conductor. In Figure 82 the direction of flow of the electrons is indicated by the arrows "dotted in," and the direction of current flow by the solidly drawn arrows. Figure 82 (a) is an illustration of electrons thrown off by a heated filament. If no means is provided for drawing the emitted electrons away from the filament they will fall back as rapidly as they are emitted and the space surrounding the filament will be filled with a constant number of electrons. If a source of DC voltage is connected across the plate and filament, so that the plate is at a positive potential with respect to the filament, electrons would be attracted to the plate and a current would be set up as shown in (b) of Figure 82. The figure shows the current flowing from plate to filament in the tube, while the electrons flow from the filament to the plate. The reason for this was discussed in section 56. If either the temperature of the filament, or the potential on the plate was varied, the flow of current would vary, but it would always flow in the same direction. Variations of the electron flow can also be obtained by placing a third element, the grid, between plate and the filament. The grid is made of very fine wire suspended with relatively large distances between the successive turns as shown in Figures 84 and 85, and is mounted closer to tile filament than to the plate. If the grid is connected to the filament battery at a point half way between the filament connections, as shown in (c) of Figure 82, so that the average difference of potential between the grid and filament is zero, no change in the plate current will be noticed. If the grid is connected at a positive potential with respect to the filament a much larger current would flow in the plate circuit, because the grid would aid the plate in drawing the electrons away from the filament. Since the grid is much closer to the filament than to the plate, its effect on the electron flow is relatively greater than that of the plate. Many of the electrons which are speeded up by the positive grid will pass between the grid wires and go to the plate as shown in Figure 82 (d), but some of them will be collected by the grid and establish a current in the grid circuit. If the grid is kept at a negative potential with respect to the filament no current will flow in the grid circuit, and the plate current will be reduced due to action of the grid in forcing some of the electrons back to the filament, as shown in Figure 82(e).

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