NEW SINGLE CHANNEL CARRIER TELEPHONE SYSTEM 179 



The stability of frequency with plate voltage variations is about 

 5/10'' parts per volt. This is adequate and was obtained without the 

 use of an expensive tuning inductance. The coil used, which also 

 serves as output and feedback transformer, has a ratio of reactance to 

 resistance of about 20 and is an air-core solenoid potted in a copper can. 



Amplifiers 



Both the receiving and transmitting amplifiers employ a single 

 pentode with about 9 db feedback. For this amount of feedback, 

 the variations of gain and impedance due to power supply variations 

 are reduced to at least one-third of the amount of the variation ob- 

 tained without feedback, and the load-carrying capacity is increased 

 about 1 db. 



The two amplifiers differ in that the frequency range transmitted is 

 different and in that the output transformer of the receiving amplifier 

 also acts as an inequality ratio hybrid coil to separate the receiving 



24 _!^ -^- +130 

 VOLTS -T- T VOLTS 



X X 



Fig. 15 — Simplified schematic of amplifier 



signaling circuit from the two-wire voice circuit. The two circuits are 

 shown in Fig. 4. In each case, the feedback is accomplished by means 

 of a bridge circuit in the output and a series connection in the input. 

 This can be more readily seen from Fig. 15, which is a simplified circuit 

 representing both amplifiers. There is a considerable saving in circuit 

 elements as compared to the familiar resistance bridge feedback con- 

 nection. The output power loss due to shunt arms of the resistance 

 bridge is eliminated. Furthermore, the impedance of the feedback 

 circuit is relatively low, and consequently some wiring difficulties were 

 avoided. In this application, the bridge is unbalanced, and the im- 

 pedance Zo is a function of KRo. As a result, it was convenient to 

 adjust Zo to the optimum value by choosing the proper value of KRq. 



