Oscillation in Tuned Radio-Frequency Amplifiers

The wide use of screen-grid tubes renders an understanding of the conditions for stability of tuned radio-frequency amplifiers important. In this paper the relation between the feed-back capacity and the other circuit and tube parameters at the threshold of instability is computed for one, two, three, and four stages. The form of the relation is found to be Ag2 CO = w) gm where g is the conductance of the load circuit and plate resistance in parallel, assuming that all circuits are similar. The values of A are 2.0, 1.0, 0.764, and 0.667, respectively, for one, two, three, and four stages. It is found that the circuits are tuned so that the phase angle of each is approximately 45 deg. lagging, still including the plate resistance of the tube in the tuned circuit. An experimental verification of the conditions for stability of a one-stage amplifier is described. The experimental value checks the theoretical value within the accuracy of the measurements.