The Tuned-Plate Tuned-Grid Oscillator

Author: J.B. Hoag

Feedback can be accomplished inside the tube through the grid-to-plate capacity.

Fig. 14 D. A tuned-plate tuned-grid oscillator

In Fig. 14 D, voltage fluctuations due to oscillations in the L₂C₂ tuned-plate circuit induce voltages directly onto the grid through the tube. These voltages are of the proper polarity to permit oscillations to be maintained. Thus, if the plate becomes slightly more positive, the voltage induced on the grid — that is, on the opposite plate of this small condenser — will be negative. This will cause a decrease in the plate current. But this lowers the voltage across the tuned-plate circuit, making the plate potential somewhat less than its static value, the reverse of the original condition. The tuned-grid and tuned-plate circuits, by virtue of their parallel resonant properties, strengthen the otherwise feeble voltage fluctuations of the grid and plate. The two circuits must be tuned to approximately the same frequency in order that oscillation shall occur. However, by slight adjustments in the tuning of either circuit, the amount of feedback, and hence the strength of the oscillations, can be adjusted. In operation, there should be no magnetic coupling between the grid and the plate circuits except that through the tube. The frequency of oscillation of this circuit is determined by the LC combination which has the higher Q.