Load Rating Theory for Multi-Channel Amplifiers * 



By B. D. HOLBROOK and J. T. DIXON 



The amplifiers of multi-channel telephone systems must be so 

 designed with regard to output capacity that interchannel interfer- 

 ence caused by amplifier overloading will not be serious. Proba- 

 bility theory is applied to this problem to determine the maximum 

 single frequency output power which a multi-channel amplifier 

 should be designed to transmit as a function of N, the number of 

 channels in the system. The theory is developed to include the 

 effects of statistical variations in the number of simultaneous 

 talkers, in the talking volumes, and in the instantaneous voltages 

 from speech at constant volume. 



Introduction 



TN A perfect multi-channel carrier telephone system, each channel 

 -^ would be entirely free from interference produced by the energy 

 present in the other channels. Since all the channels are amplified 

 by the same repeaters, which as a practical matter cannot have per- 

 fectly linear characteristics, this is an ideal that may be approached 

 but not completely realized. The interchannel interference must be 

 kept down to a value which will be satisfactory for the grade of transmis- 

 sion concerned, further reduction being uneconomic. To do this the 

 repeaters must meet definite load capacity requirements and modula- 

 tion (non-linearity) requirements. The load capacity requirement is 

 most conveniently specified in terms of the maximum single frequency 

 sine wave power which a multi-channel amplifier must transmit with- 

 out appreciable overloading. The modulation requirement pertains 

 to the performance of the amplifier for impressed loads equal to or 

 smaller than the load capacity, and specifies the allowable power in 

 the modulation products resulting from such loads. Because of the 

 numerous factors which affect these requirements, their determination 

 is a rather complicated matter and the present discussion will be 

 restricted solely to a determination of the load capacity requirement. 

 The object is to determine this quantity as a function of N, the number 

 of channels in the system. 



The criteria ordinarily used for determining the load capacity of 

 single-channel amplifiers are of little use here because of two funda- 



* Presented at Great Lakes District Meeting of A.I.E.E., Minneapolis, Minn., 

 September 27-29, 1939. 



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