262 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



open the gate the back bias is reduced to a small reverse voltage which is 

 more than overcome by the signal pulse amplitude of the pulse. The 

 pulse thus forward biases the diode and is transmitted to the output. 



The inhibiting gate has its diode normally in the conducting state so 

 that a pulse is readily transmitted from input to output. When the bias 

 is changed the diode is heavily back biased so that the pulse amplitude 

 is insufficient to overcome this bias. 



The elements of 12 gates are mounted on a single printed wiring board 

 w4th plug-in terminals and a metal enclosure as shown in Fig. 2(a). All 

 elements are mounted in one side of the board so that the opposite side 

 may be solder dipped. After soldering the entire unit (except the plug) 

 is dipped in a silicone varnish for moisture protection. 



b. Transistor Bistable Circuit 



Transistors are inherently well adapted to switching circuits using but 

 two states, on (saturated) or off.^^ In these circuits with a current gain 

 greater than unity a negative resistance collector characteristic can be 

 obtained which will enable the transistor to remain locked in its conduct- 

 ing state (high collector current flowing) until turned off (no collector 

 current) by an unlocking pulse. At the time the concentrator develop- 

 ment started only point contact transistors were available in quantity. 

 Point contact transistors have inherently high current gains (>1) but 

 the collector current flowing when in the normal or unlocked condition 

 (Ico) was so great that at high ambient temperatures a relay once op- 

 erated in the collector circuit would not release. 



Junction transistors are capable of a much greater ratio of on to off 

 current in the collector circuit. Furthermore their characteristics are 

 amenable to theoretical design consideration.^^ However, the alpha of a 

 simple junction transitor is less than unity. To utilize them as one would | 

 a point contact transitor in a negative resistance switching circuit, a 

 combination of n-p-n and p-n-p junction transistors may be employed, i 

 see Fig. 9(b). Two transistors combined in this manner constitute a ' 

 "hooked junction conjugate pairs." This form of bi-stable circuit was j 

 used because it requires fewer components and uses less power than an 

 Eccles-Jordan bistable circuit arrangement. It has the disadvantage of a 

 single output but this was not found to be a shortcoming in the design 

 of circuits employing pulse gates of the type described. In what follows 

 the electrodes of the transistor will be considered as their equivalents 

 shown in Fig. 9(b). 



The basic bi-stable circuit employed is shown in Fig. 10. The set 



