Grid Control of the Space Charge

Author: J.B. Hoag

A triode or three-electrode tube contains a mesh or grid of wires in addition to the filament and plate of a diode. As usually operated, the grid, located between the filament and the plate in the middle of the space charge, is negatively-charged with respect to the filament and hence adds its effect to that of the space charge in limiting the number of electrons which flow to the plate.

Fig. 12 A. Circuit used to plot the characteristic curves of a triode. (From E. & N. P.)

Fig. 12 A shows the circuit used to obtain the family of characteristic curves shown in Fig. 12 B. When the grid is made very negative, it strongly repels electrons, prevents them from getting over to the plate, and hence keeps the plate current I_p down to a very small value. If, however, the grid is less negative, more current flows in the plate circuit as shown by the rise in the curves in Fig. 12 B.

Fig. 12 B. Characteristic curves of a triode. Plate current Ip vs. grid voltage Eg (From E. & N. P.)

In this figure, the different curves correspond to the different voltages on the plate; the upper curves when the plate voltage is large, the lower curves for the cases when it is small.

The characteristic curves of a tube prove very useful both in understanding the principles involved in the applications of the tubes and in the design of electronic circuits. Such curves may be obtained easily for a given tube in the laboratory or they may be obtained from data sheets supplied by the manufacturer of the tube. Note that the curves are essentially straight over their central portion, and that they have a longer straight region when the plate voltage is high. Note also that they bend at their lower left or cutoff end and at their upper right or saturation end.