TABLE OF CONTENTS In the last two chapters, we studied two principal methods by which a-f signals may be amplitude-modulated upon r-f carriers. A great many transmitters perform this modulation in the final r-f stage by one of these methods. It is often desirable, however, especially in transmitters of quite high power, to impress the audio upon the carrier in one of the low-power stages and then to amplify the signal by means of one or more linear-amplifier stages. A linear amplifier is one whose r-f output voltage is directly, or linearly, proportional to the r-f excitation voltage; when it has this property, it can amplify without distortion a carrier which is already modulated.
It is fairly obvious that a class A amplifier is capable of doing this job. If we bias an r-f amplifier properly for class A operation and then begin to apply r-f voltage to the grid of the tube, we shall find that the r-f plate voltage will increase in direct proportion to the r-f grid voltage as we raise the grid drive. At some point, saturation of the tube will occur; if we make note of the grid-drive voltage at this point and then reduce it to one-half this value, we shall have arrived at the correct amount of drive for carrier-level condition. If this grid excitation is then amplitude-modulated 100 per cent, the r-f plate voltage will rise to twice carrier-level voltage for the positive peak of modulation and will decrease linearly to zero...
