1112 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 195G 



currents,* permits the use of a simple input circuit. The AND type input 

 circuit is suitable and desirable for another reason. When AND type 

 logic is added to the amplifier, it may be paralleled with the basic input 

 circuit and the input sensitivity of the complete network will be the same 

 as for the amplifier alone. Other logic circuits will be added to an ampli- 

 fier in a manner similar to that described by Felker- so that the input 

 sensitivity will be reduced at most by the voltage drop across one series 

 diode (approximately 0.3 volts). 



The input pulse voltage and current requirements depend upon the 

 voltage threshold necessary to prevent false operation and the minimum 

 trigger current for reliable regeneration. A test of several sample tran- 

 sistors indicates that approximately 0.3-ma emitter current is required 

 to trigger the transistor with an estimated collector supplj^ voltage of 

 10 volts. The emitter breakpointf voltage is found to vaTy between 

 — 0.25 and +0.25 volts. To allow for aging variations of the transistor 

 and of R2, it seems reasonable to use a 6-volt source and R2 ec^ual to 

 9090 ohms, which results in a trigger current a little more than twice the 

 required minimum. Previous experience with computers of this type in- 

 dicates that a 2-voIt threshold will be sufficient to prevent false trigger- 

 ing. Thus, the secondary winding of the feedback transformer is returned 

 to —2 volts and Rl is chosen to give a quiescent emitter voltage of —2 

 volts. With these considerations and an estimated voltage drop across 

 R3, the input pulse amplitude is calculated to be 2.3 volts and 0.9 ma. 

 Allowing 0.3 volts for a series logic diode, the minimum output voltage 

 and current of the amplifier are 2.6 volts and 0.9 ma per driven network. 



The selection of the collector supply voltage and the turns ratio of T2 

 depends upon the dc power dissipation due to Ico current and output 

 voltage regulation versus collector current. For this transistor a unity 

 turns ratio appears to represent a reasonable compromise. Then, by 

 estimating the voltage drops across Tl, T2, and the transistor, it is 

 found that a collector supply voltage of — 8 volts is suflScient to produce 

 an output pulse voltage about 0.5 volt greater than the required 

 miminum. 



The next step is the selection of the turns ratio of Tl and the primary 

 inductances of both Tl and T2. The two considerations involved are 

 sufficient feedback with the minimum output current (the worst case 

 with respect to feedback) and the maximum collector dissipation in the 

 event that the clock fails. By means of the formulas and assumptions 

 indicated in section 5, primary inductance values of 0.4 mh for Tl and 



* Usually a > 4 for ie = 0.5 ma. 



t The transition point of the emitter diode from cut-ofT to conduction. 



