356 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 19o7 



of the circuit in much the same manner as for electron tube circuits. One 

 significant difference is that in this transistor circuit only the second 

 transistor introduces a 180-degree phase shift. This permits both input 

 and output coils to return to a common ground as in a three-tube electron- 

 tube circuit and thereby avoids the circuit complications of a two-tube 

 circuit where one of the coils must float off ground. A simple resistance 

 interstage is used and battery filtering completes the circuit. 



3.3 Low Voltage Protection 



Use of transistors gave rise to the need for supplementary protection 

 from voltage surges on the line below those for which conventional carbon 

 blocks afford protection. This additional protection was obtained by 

 using the reverse voltage breakdown characteristics of newly developed 

 silicon-aluminum junction diodes as shown in Fig. 4. Protection is pro- 

 vided in the 50- to 1,000-volt range by the diodes and above a nominal 

 value of 750 volts by the carbon blocks. During the normally short period 

 of operation the small diodes carry a current of up to 10 amperes. 



3.4 System Levels and Carrier Line Loss 



The carrier frequency output power of the transistorized transmitting 

 amplifier in the terminals was set at -|-6 dbm. This level was limited 

 primarily by the power handling capabilities of the transistors used. Be- 

 cause of the loss of the secondary protection circuitry, band filters, and 

 the line transformer, this became +4 dbm at the carrier line terminals. 

 This is equal to the highest carrier power transmitted by the Type Nl 

 carrier system. With 50 per cent modulation of the carrier, the effective 

 sideband level at the transmitting line terminals is only 2 db below that 

 transmitted by the Type carrier system, the most recent carrier system 

 used for open-wire long distance trunks of the Bell System. 



TO 



CARRIER 



EQUIPMENT 



CURRENT-LIMITING 

 RESISTOR 



V\A — 



SECONDARY 



PROTECTION 



DIODES 



\ 



TO 



CARRIER 



LINE 



CARRIER LINE 

 TRANSFORMER 



Fig. 4 — Secondary protection circuit in carrier portion of PI carrier terminal. 



