GB471423A - Improvements relating to thermionic valve circuits - Google Patents

Abstract

471,423. Valve amplifying circuits. STANDARD TELEPHONES & CABLES, Ltd. (Western Electric Co., Inc.) March 3, 1936, No. 6416. [Class 40 (v)] An amplifying system for modulated carrier waves is increased in efficiency by providing two parallel amplifying paths, one supplying power to the load only when the carrier voltage is above a normal value, the other supplying power at all carrier voltages but having a flat output characteristic for voltages above the normal. Phasing networks are arranged in one or both paths so that the carrier is in phase quadrature at the respective amplifiers but is brought into phase again before recombination in the load circuit. As shown in Fig. 9, the two amplifiers 1, 2 are fed from adjustable tappings on a potentiometer 9 across the source S of modulated carrier waves, the valve 1 being biassed to platecurrent cut-off point, and the valve 2 being more strongly biassed. A filter section 4, gives a phase shift of 90‹ to the input to valve 2, the plate of which is connected directly to the load resistance R. The other valve 1 has a similar filter section 5 on its output side, so that the carriers are in phase in the load R. When the input voltage exceeds the normal, the grid of valve 1 begins to take current 20 reducing the effective input voltage, and increasing the bias. As the valve 2 begins to deliver current to the load, it modifies the impedance into which the valve 1 is working, and so contributes to the flattening of its output voltage characteristic. The filters 4, 5, may be of the band-pass, lowpass, or high-pass type, and may each comprise one or more sections. Both filters may be associated with the amplifier 1, Fig. 10 (not shown). In Fig. 11, the amplifiers 1, 2 are fed in push-pull from the source S, and the plate circuit of the amplifier 2 is back-coupled through a transformer 20 and phase-shifter 21 to a common branch of the grid circuits ; the effect being that when the carrier voltage is above normal, the feed-back comes into action and opposes the input voltage in amplifier 1 but assists in amplifier 2. The filters 3, 5 of Fig. 11 are such that together they displace the carrier by 180‹, so that the outputs of valves 1, 2 are in phase in the aerial. In the circuit shown in Fig. 12, a degenerative feed-back connection 29 is made from the load circuit 18 to the grid circuit of an amplifier 25 interposed between the source 5 and the double transmission path 24. The phase of the feed-back is controlled by a coil 30 and condenser 31. The load circuit may be connected in series relation with the output circuits of the two amplifiers 1, 2, Figs. 1 and 1a (not shown). The Specification includes an analysis of a typical circuit, and diagram illustrating the output voltages and currents for the respective valves, plotted against the input voltage.