Radio Broadcast (Nov 1923-Apr 1924)

Record Details:

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300 Radio Broadcast element vacuum tube as an amplifier, by showing the relationship between the plate current and the grid voltage, when the plate voltage and the filament current are held constant. The vertical line on the extreme left indicates the value of plate current (usually expressed in milliamperes) ; the horizontal line, on the bottom, the grid voltage; the vertical line in the center, the zero line, i.e., the dividing line between negative and positive grid voltage. To the right of this line is positive grid voltage and to the left, negative. The curve shows the variation of the plate current, and the bend X is the saturation point. Assume that the curve illustrates the characteristics of a UV-201-A as an amplifier with a plate voltage of 67J volts. The dot on the curve and point D on the left-hand line indicates the normal plate current as explained in the preceding paragraph. Now suppose that an incoming signal of a medium intensity has been impressed upon the grid. The applied potential being of alternating character, an alternating positive and negative charge is therefore applied to the grid. This is shown by YJ as the negative charge and YK as the positive charge. By following up the vertical lines at these points and noting where they cross the plate current curve then following the horizontal line at that point toward the left vertical line, we see the value of the plate current in each case. The negative charge upon the grid has reduced the plate current a certain value below normal, and the positive charge upon the grid has increased the plate current the same value above normal. This phenomenon occurs because when there is a negative charge upon the grid it repels some of the electrons emitted from the filament and less travel to the plate, hence the plate current is reduced: the opposite is true when the grid is charged positively. The grid attracts electrons and a greater number travel to the plate, increasing the plate current. Expressing this differently, we can say that when the grid carries a positive charge it increases the attracting power of the plate. By glancing once more at the curve it will be noted that the increase and decrease in the plate current are exactly proportional to the grid potential variations, but are of greater magnitude; so the wave Do you know just how your tube works? What difference does it make if you apply more B battery, or more filament voltage? Mr. Rider answers these questions in this article. form in the plate circuit will be exactly the same as those in the grid, and no distortion will take place in this case. Let us now assume that the signal has passed through one stage of amplification, or that the original input has been greatly increased. What happens when a stronger signal is applied to the grid? Since the signal is of greater intensity on being amplified, the alternating potential that is impressed upon the grid is greater. This is indicated by YI and YL, the former the negative charge and the latter the positive. We follow the various lines through once more, and note that the negative half of the applied grid potential has produced the plate current variation DG, and the positive half the variation DA, but that the two variations are not equal and therefore are not exactly proportional to the grid potential variations. The reduction in the plate current for the negative charge on the grid is greater than the increase for the positive charge, and the increase has been carried beyond the saturation point. Since the variations in the plate current are not exactly equal, the wave-form in the plate circuit is not an exact reproduction of that applied to the grid. The effect of this is distortion. Thus distortion is produced within the vacuum tube in amplifying circuits. What is the remedy for this situation? To reduce the grid voltage would reduce the amplification. That would never do, for maximum amplification is generally desirable; therefore, we must reduce the normal plate current to such a point that equal increase and decrease will be produced in it by the grid voltage variations. Expressed differently, the operating point (normal plate current) must be shifted to such a position that the plate current variations will be equal at all times, and this point is midway between the two bends. We can do this by two direct methods: 1. By reducing the filament brilliancy, thus reducing the electronic emission. 2. By reducing the plate voltage, thus reducing the attracting power of the plate. Neither method is completely desirable, for in each case we would not obtain maximum amplification. We shall have to resort to an indirect method of shifting the operating point