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BELL SYSTEM TECHNICAL JOURNAL 



have been built most recently for use in these systems are known as "3-megacycle 

 amplifiers" and were intended to provide about a 2-megacycle band of suitable 

 characteristics for telephone purposes or about a 2f megacycle band suitable for 

 television transmission. 



Figure 4 shows one of these amplifiers. It is a three-stage feedback device 

 using two small pentodes in parallel in each stage. The mathematical design of 

 the circuit is beyond the scope of this paper and has been treated elsewhere.* 

 This type of pentode has an initial transconductance of from 2000 to 2500 mi- 



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25 



5 20 

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FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 5 — Repeater gain line loss and equalization characteristics 



cromhos and an output power of .1 to .2 watt at 130 volts as used in this system. 

 These tubes are in parallel only to give added reliability. The gain of this am- 

 plifier is very roughly the complement of the line loss as a function of frequency. 

 With this amplifier and the cable described above, these repeater sections are about 

 5^ miles in length. As illustrated in Fig. 5, the difference between the gain and 

 line loss is made up by a line equalizer so that to a first approximation, zero loss 

 in transmission is obtained at all frequencies within the band over each repeater 

 section. About 30 db of feedback is effective over the telephone frequency band 

 (i.e. up to 2000 kc) around the entire amplifier with about 10 db additional around 



