1080 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



time under relatively special operating conditions, consistently good 

 performance is demanded at all times, even with wide variations of cir- 

 cuit parameters and operating conditions (as, for example, a twenty-to- 

 one variation in the required output current). Therefore, various circuit 

 possibilities will be examined from the standpoint of reliable per- 

 formance. 



When the switching and mathematical operations of a digital data 

 processing system are accomplished by a network of passive logic cir- 

 cuits with amplifiers interspersed to overcome circuit losses,^- ^ the 

 environment of an amplifier is generally as indicated in Fig. 1. The 

 signal information that passes from one logic network to another is 

 represented in a code by a group of discrete pulses. Due to the nature 

 of this digital information, utmost reliability of each amplifier is an 

 important requirement that greatly influences the amplifier design. 

 Since the position of a pulse in time or place determines its significance 

 to the system, it is necessary that each pulse be identically amplified 

 and that noise or extraneous disturbances do not cause false output 

 pulses from an amplifier. The effect of an error or a failure in operation 

 is different for different systems and in a given system depends upon 

 the time or place of the failure. In some computers a single mistake will 

 invalidate an entire computation cycle, while a permanent failure of 

 even a single amplifier will cause complete system failure in almost any 

 digital machine. Experience with the type of amplifier under discussion 

 indicates that failure rates of less than a tenth of one percent per thou- 

 sand hours are attainable. 



Jf 



LOGIC 



LOGIC 



t! 



LOGIC 



DELAY 



u 



it 



LOGIC 



U 



LOGIC 



It 



LOGIC 



LOGIC 



n 



DELAY 



Fig. 1 — Typical environment of an am])lifier. 



