Grid Modulation

Of all the useful systems for producing an AM wave suitable for radio transmission, the grid-modulated amplifier is the simplest with regard to the amount of equipment involved. In this system, the bias voltage applied to the grid of an r-f amplifier tube is varied at the audio rate, while the r-f excitation and the plate voltage applied to the tube are kept constant.

It is not difficult to see how modulation is accomplished by this method if we trace through the conditions required for this type of operation. To begin with, let us suppose that we are operating a class C amplifier, with a bias voltage of 2.5 times cutoff for the operating plate voltage and with just sufficient r-f excitation applied to the grid to drive the tube to the condition of saturation. This will produce the maximum amount of power that the tube is capable of delivering to a given plate load with a given plate voltage. The power thus produced will therefore be the amount we can expect at the crest of the modulation cycle. Now, if we increase the negative value of the bias voltage without changing the r-f excitation voltage, the grid will not be carried so far in the positive direction during its r-f swing (see Fig. 60-1). Both the r-f plate-voltage swing and the d-c plate current will be reduced by this change, and in the same proportion. At some particular value of bias, the r-f output voltage and...