884 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



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FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 2 — Required line amplifier gain. 



plus gain shaping and impedance adjusting elements. Since there is no 

 feedback around the coupling networks, they directly affect the insertion 

 gain of the amplifier, as do the two beta circuits and the regulating net- 

 work. The required shaping of insertion gain across the transmitted band 

 is obtained and controlled by the design of these five networks. Fig. 2 

 shows the required amplifier gain for nominal and extreme thermistor 

 settings; these required gains differ from the line loss by the small 

 losses of the associated repeater components. At mid-range thermistor 

 setting about 37 db of gain shaping is needed. The manner in which this 

 shaping is distributed among the five networks has important effects 

 on the feedback which can be obtained and the sensitivity of amplifier 

 gain to element variations. 



This configuration offers several advantages, one of the most im- 

 portant of which is that the regulating network is between the amplifiers. 

 In most other configurations, the only gain-determining network avail- 

 able for the regulating function is the beta circuit. When the feedback 

 is not infinite, this introduces errors for which it is difficult to compensate, 

 and limits the available feedback by complicating the design. In this con- 

 figuration, the impedances which the amplifiers effectively present as 

 shunts across the regulating network are high and can be allowed for 

 in the design so that the regulation error can be made much smalloi' 

 than the error associated with beta circuit regulation in an amplifier 

 having relatively little feedback. Other major advantages are the superior 

 signal-to-noise performance and the relative simplicity of the feedback 

 loops of this amplifier as compared to alternative designs. 



