The Bell System Technical Journal 



January, 1934 



Stabilized Feedback Amplifiers* 



By H. S. BLACK 



This paper describes and explains the theory of the feedback principle 

 and then demonstrates how stability of amplification and reduction of 

 modulation products, as well as certain other advantages, follow when 

 stabilized feedback is applied to an amplifier. The underlying principle 

 of design by means of which singing is avoided is next set forth. The paper 

 concludes with some examples of results obtained on amplifiers which have 

 been built employing this new principle. 



The carrier-in-cable system dealt with in a companion paper ^ involves 

 many amplifiers in tandem with many telephone channels passing through 

 each amplifier and constitutes, therefore, an ideal field for application of 

 this feedback principle. A field trial of this system was made at Morris- 

 town, New Jersey, in which seventy of these amplifiers were operated in 

 tandem. The results of this trial were highly satisfactory and demon- 

 strated conclusively the correctness of the theor>' and the practicability 

 of its commercial application. 



Introduction 



DUE 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. 



' "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. 



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