Phase and Voltage Considerations

Author: J.B. Hoag

There are three necessary conditions for oscillation: (1) an amplifying device such as a vacuum tube; (2) sufficient yet not too much feedback action; (3) proper polarity of the feedback.

In order to understand the phase or polarity relationships, let us imagine that the grid of an oscillating tube becomes more positive than its normal d.c. operating value. This will cause an increase in the plate current. This causes a potential drop across the plate load (for example, the tickler coil of the circuit shown in Fig. 14 A). The end of the load connected to the plate of the tube will become less positive. When the grid potential goes toward positive, the plate potential decreases, and vice versa. This may be stated in different words: namely, the grid and plate fluctuating voltages are 180° out of phase with respect to each other. This is illustrated in Fig. 29 K.

Fig. 29 K. Voltages on the plate and grid of an oscillating vacuum tube

It is to be recalled that the plate current is a maximum at the same instant that the plate voltage is a minimum. It is at this time that energy from the plate battery is given up to the oscillating circuit. It is also to be noted that the grid of the oscillator becomes positive, and that a certain amount of grid current flows. For optimum operating conditions, the peak value of the grid voltage should amount to approximately 80 percent of the minimum value of the plate voltage.