Multi-channel radio-frequency amplifiers

The economic advantages of wide-band multi-channel transmission either by radio or high-frequency cable links were appreciated early in the war, and systems of this type were developed on a wide scale. The simultaneous amplification of the channel signals without introducing noticeable distortion or mutual interference presented highly specialized problems which were solved economically in the majority of cases only by the use of negative feedback. The most outstanding contribution to amplifier design during the period 1939¿1945 was the analysis by H. W. Bode of feedback amplifier networks which established the design procedure on a firm logical basis. No appreciable improvement took place in valves for wide-band amplifiers up to 50 Mc/s, and it would appear that a near-maximum amplifier performance is now being obtained with existing valves so that the next major development might be expected in the thermionic field. Many different types of amplifier were developed during the war period, usually to meet urgent specific demands, and a representative selection is described briefly in the paper. The examples range from a highly stable (60-db feedback) 100-mW repeater for speech channels on a coaxial cable, to a transmitter amplifier capable of feeding up to 12 simultaneous 40-watt telegraph signals in the frequency band 3¿8 or 8¿15 Mc/s direct into a rhombic aerial.