Stabilized Feedback Amplifiers

D UE TO advances in vacuum tube development and amplifier technique, it is now possible to secure any desired amplification of the electrical waves used in the communication field. When many amplifiers are worked in tandem, however, it becomes difficult to keep the overall circuit efficiency constant, variations in battery potentials and currents, small when considered individually, adding up to produce serious transmission changes for the overall circuit. Furthermore, although it has remarkably linear properties, when the modern vacuum tube amplifier is used to handle a number of carrier telephone channels, extraneous frequencies are generated which cause interference between the channels. To keep this interference within proper bounds involves serious sacrifice of effective amplifier capacity or the use of a push-pull arrangement which, while giving some increase in capacity, adds to maintenance difficulty. However, by building an amplifier whose gain is deliberately made, say 40 decibels higher than necessary'(10,000 fold excess on energy basis), and then feeding the output back on the input in such a way * Presented at Winter Convention of A. I. E. E., New York City, Jan. 23-26, 1934. Published in Electrical Engineering, January, 1934. 1 "Carrier in Cable" by A. B. Clark and B. W. Kendall, presented at the A. I. E. E. Summer Convention, Chicago, 111., June, 1933; published in Electrical Engineering, July. 1933, and in Bell Sys. Tech. Jour., July, 1933.