The Tube as a Generator

Before going into the practical application of r-f amplifiers for the production of radio signals, it is very important that we develop a good understanding of the relationship of the tube to its plate load impedance. Let us put aside the idea that the tube is an amplifying device and think of it instead as a generator of r-f energy. The tube is simply a source which applies a certain r-f voltage across the terminals of a tank circuit, and the output power and some other characteristics of the signal will be dependent upon the circuit conditions of this tank circuit.

The class C amplifier is ordinarily operated in a saturated condition; this means that the excitation, or signal input, to the grid is sufficiently large that no increase in the tube's output can be obtained by increasing the grid excitation. At lower levels of excitation, the amplitude of the r-f swing in plate voltage is dependent upon the amplitude of the swing in grid voltage; but as the excitation is increased, the plate swing finally arrives at a value beyond which it cannot go. This limit occurs when the peak value of the r-f plate voltage is equal to about 90 per cent of the d-c plate voltage applied. Accordingly, if the tube is driven to this saturation condition, we know that the peak r-f voltage across the tank circuit will be 0.90 times the d-c voltage and that its rms value will be...