The Screen-Grid Tube

Radio-frequency amplification by means of the three-electrode tube is usually disappointing. With resistance coupling the feedback through the tube is in such a phase relation to the input voltage as to reduce the amplification below that given by the simple equation for voltage amplification. With impedance coupling the feed-back causes self-oscillations when the circuit conditions are such as might be expected to give large amplification. In the shielded grid tube the feed-back is reduced to a negligible amount and the current through the tube is very nearly independent of the plate voltage over the working range. Under these conditions, the voltage amplification becomes the product of the mutual conductance and the load impedance. High impedance in the plate circuit is obtained by using a sharply tuned parallel circuit. With proper shielding such a circuit may be used without producing self-oscillation. At a frequency of 700,000 cycles per second, amplifications of 80 fold per stage may be obtained.