Radio Broadcast (Nov 1923-Apr 1924)

How the C Battery Prevents Distortion 301 ui FIG. 3 Showing the " characteristic curve" when C battery (grid bias) is used GRID VOLTAGE of the tube. We shall utilize the grid, we can either increase or decrease the plate current. As we want to reduce the normal plate current to a certain point, we place a constant negative charge upon the grid. This calls for the insertion of a source from which we can obtain this negative charge. The most logical source would be a battery, and that is the means employed in actual practice. This battery is called the C battery, and is sometimes known as the "grid bias." The battery is connected as shown in Fig. 4, and we apply just enough negative potential to the grid to reduce the normal plate current to the point where changes in the grid potential due to the incoming signal, will produce equal increases and decreases in the plate current. Therefore, by means of the C battery, we can confine ourselves to operation on the straight portion of the characteristic curve, and place the operating point midway between the two bends. In this way, the symmetry of the oscillations applied to the grid are faithfully reproduced in the plate current, the variations are proportional, and distortion is eliminated. Further, the greatest amount of variation is obtained in the plate current for the grid potential variation, and maximum amplification is also obtained. This is shown in Fig. 3. The operating point, R, has been reduced, and for the same value of input the greatest variation is obtained and the variations are proportional. Fig. 4 is the same as Fig. 1, except for the C battery between the grid return lead of the transformer and the negative terminal of the A battery. That point is the proper position for the battery, and not between the grid of the tube and the grid terminal of the transformer. The patent rights for the use of a C battery in an amplifier are owned by certain radio concerns, and because of their refusal to grant other manufacturers this right, separate C batteries are not found in the average amplifying unit. There is no ruling that forbids the radio fan to insert such a battery in his own amplifier. To overcome this patent situation, other methods of obtaining a negative grid bias are resorted to by the manufacturers. One is to connect the grid return of the transformer to the negative terminal of the A battery, and another to place the filament rheostat into the negative lead of the A battery and connect the grid return of the transformer to a part of the rheostat, so as to apply the voltage drop across the rheostat to the grid of the tube. These methods are limited in their degree of efficiency, since the required value of "grid bias" might be more than that obtainable by the above means. The value of the C battery is best determined by experiment, as it depends upon the controlling constants of the tube. The following table offers a close approximation to the C battery voltages to be used with a given plate voltage. PLATE VOLTAGES 40 60 80 IOO 120 C BATTERY VOLTAGE 0.5 to 1.0 1.0 to 3.0 3.0 to 4.5 4.5 to 6.0 6.0 to 9.0 (not to be used with UV-199's or C-299's) The C battery: 1 . Eliminates distortion. 2. Reduces the normal plate current, thus increasing the life of the B battery and making the amplifier more economical to operate. This is illustrated by the following experiments: + RMR B FIG. 4 Showing how the C battery is connected in an audio-frequency amplifier