TOLL TRANSMISSION IN THE UNITED STATES 



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of transmission. The total amplification is affected by the require- 

 ment that the New York-Chicago circuit is expected to have a net 

 attenuation of not over 9 db, and to be stable within about ± 2 db. 

 These figures may seem large and the requirements difficult to 

 meet, but with the systems under development, the magnitude of the 

 high-amplification problem is even greater. In the carrier on cable 

 development, the circuits will consist of non-loaded pairs, and it will 

 be necessary to so space the amplifiers and adjust their amplification 

 that the total amplification on, for example, a New York-Chicago 

 circuit will be about 3000 db at the center of the frequency band, or a 

 power ratio of 10^"". Obviously, the stability requirement has been 

 made much more rigid. With a typical coaxial circuit, the overall 

 amplification at a million cycles for a thousand-mile circuit (1600 km.) 

 may well be 6500 db or a power ratio of 10^^''. 



Fig. 9 — Simplified schematic diagram of a feedback amplifier. 



Furthermore, with the relatively simple circuit shown in Fig. 7, the 

 amplifiers are called upon to handle merely the currents corresponding 

 to one telephone conversation, while in ,the broad-band system an 

 amplifier is required to handle simultaneously a large number of 

 carrier telephone channels. To avoid the generation of extraneous 

 frequencies or intermodulation products which would cause inter- 

 ference between the channels, an amplifier must be adopted which is 

 more nearly perfect in this respect than any heretofore standard. 



This problem of amplifier stability and perfection was solved some 

 little time ago by an invention of one of our engineers. This engineer 

 devised a new amplifier circuit which has been termed the "stabilized 

 feedback amplifier." Some older types of amplifiers took some of 

 their output and fed it back to the input for the purpose of increasing 

 the amplification. This new feedback circuit controls the phases of 

 the currents in the amplifier and feedback circuits so that the amplifi- 

 cation is decreased. As a result, we have available an amplifier which 

 is remarkably stable and closely linear in its performance. Figure 9 



