TABLE OF CONTENTS The most widely used of all modulation methods in radio transmission is that of plate modulation. Here the audio voltage is introduced in series with the d-c plate voltage of the tube, with such amplitude that the plate voltage will be driven down to zero and up to twice the unmodulated value in the application of 100 per cent modulation. Doubling the plate voltage at the modulation crest likewise doubles the plate current, thus giving a peak input equal to four times the unmodulated carrier input. Therefore, if the efficiency remains substantially constant over the modulation cycle, we shall obtain the required output of four times carrier power at the positive peak of modulation. We can provide these conditions of efficiency by seeing to it that the tube stays in saturated class C conditions of operation at all times. Accordingly, we must provide a negative bias of about four times cutoff for the unmodulated d-c plate-voltage value, so that, when the plate voltage is doubled at the modulation crest, the bias will still be about two times cutoff.
Since the efficiency of this amplifier is kept at its highest possible value over the entire modulation cycle, we can see that the carrier efficiency will be much higher than that of the grid-modulated amplifier. Whereas the grid-modulated tube will operate at a carrier efficiency of about 37.5 per cent, we may expect the plate-modulated tube to give us a carrier efficiency of some 75 per cent. In view of...
