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Anode current in the lamp plays the same role as the collector current of transistor: Using battery power, it creates a "powerful copy" of the amplified signal. However, the current control of the lamp is carried out not as a transistor.
In the transistor collector current is changed because under the influence of the amplified signal varies the number of charges, which go from the emitter through the base and get into the collector circuit. If we wanted the same manner to control the anode current in the lamp, we would have to pass through the cathode current is amplified so that the action of this current varied the temperature of the cathode, and, consequently, the number of electrons emitted from a. Of course, it is almost unusable at least, because the amplified signal is usually too weak and cannot heat the cathode. In addition, due to the thermal inertia of the cathode (for heating and cooling of the cathode takes some time) a change in its temperature will not keep pace with changes in the amplified signal.
To control the anode current in the lamp with the third electrode - a metal mesh, which is very close to the cathode? Therefore, if between the grid and the cathode acts, even a small voltage, it is a very strong effect on the anode toga: In many tubes enough to apply for a negative voltage grid 5-10 which repels electrons back to the cathode to the anode current has stopped, despite the attractive effect of a fairly large (usually 50-250 in) a positive voltage on the anode. In this case, we say that the lamp voltage grid locked.
The less negative voltage on the grid, the weaker it repels the electrons, the greater their number, slipped the net, goes to the anode, and consequently, the more plate current. For positive voltages on the grid, it is not only frustrating but even helps the movement of electrons to the anode, thereby increasing the anode current.
1 When talking about voltage on any electrode lamps, such as a grid or anode, then keep in mind that this voltage is measured relative to the cathode. Sometimes, for brevity, they say, "minus the grid" or "plus at the anode", referring to the positive or negative voltage to the respective electrodes relative to the cathode. |