. The Bell System technical journal . negative coefficient of the papertuning condenser. NEW SINGLE CHANNEL CARRIER TELEPHONE SYSTEM 179 The stability of frequency with plate voltage variations is about5/10 parts per volt. This is adequate and was obtained without theuse of an expensive tuning inductance. The coil used, which alsoserves as output and feedback transformer, has a ratio of reactance toresistance of about 20 and is an air-core solenoid potted in a copper can. Amplifiers Both the receiving and transmitting amplifiers employ a singlepentode with about 9 db feedback. For this amount
. The Bell System technical journal . negative coefficient of the papertuning condenser. NEW SINGLE CHANNEL CARRIER TELEPHONE SYSTEM 179 The stability of frequency with plate voltage variations is about5/10 parts per volt. This is adequate and was obtained without theuse of an expensive tuning inductance. The coil used, which alsoserves as output and feedback transformer, has a ratio of reactance toresistance of about 20 and is an air-core solenoid potted in a copper can. Amplifiers Both the receiving and transmitting amplifiers employ a singlepentode with about 9 db feedback. For this amount of feedback,the variations of gain and impedance due to power supply variationsare reduced to at least one-third of the amount of the variation ob-tained without feedback, and the load-carrying capacity is increasedabout 1 db. The two amplifiers differ in that the frequency range transmitted isdifferent and in that the output transformer of the receiving amplifieralso acts as an inequality ratio hybrid coil to separate the receiving. 24 _!^ -^- +130VOLTS -T- T VOLTS X X Fig. 15—Simplified schematic of amplifier signaling circuit from the two-wire voice circuit. The two circuits areshown in Fig. 4. In each case, the feedback is accomplished by meansof a bridge circuit in the output and a series connection in the can be more readily seen from Fig. 15, which is a simplified circuitrepresenting both amplifiers. There is a considerable saving in circuitelements as compared to the familiar resistance bridge feedback con-nection. The output power loss due to shunt arms of the resistancebridge is eliminated. Furthermore, the impedance of the feedbackcircuit is relatively low, and consequently some wiring difficulties wereavoided. In this application, the bridge is unbalanced, and the im-pedance Zo is a function of KRo. As a result, it was convenient toadjust Zo to the optimum value by choosing the proper value of KRq. 180 BELL SYSTEM TECHNICAL JOURNAL
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