32G 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



of a free running blocking oscillator after the input voltage Vi passes 

 through the reference level. After a period of time the transistor will 

 return to the "on" state unless the voltage Vi is sufficiently large at this 

 time to prevent switching. In order to minimize the required slope of the 

 hiput waveform the time interval between the instant Vi passes through 

 the reference level and the instant the transistor would naturally switch 

 to the "on" state must be maximized. This time intei-val can be con- 

 trolled by connecting a diode D3 across the secondary winding of trans- 

 former Ti . When the transistor turns off, the current which was flowing 

 through the secondary of transformer Ti(Ic) continues to flow through 

 the diode D3 so that L2 and D3 form an inductive discharge circuit. The 

 point contact diode D3 has a forward dynamic resistance of less than 10 

 ohms and a forward voltage drop of 0.3 volt. If the small forward re- 

 sistance of the diode is neglected, the time required for the current in the 

 circuit to fall to zero is 



T = 



0.3 



(28) 



During the inductive transient, 0.3 volt is induced into the primary of 

 transformer Ti (since N = 1) maintaining the transistor cutoff. The 

 duration of the inductive transient can be made as long as desired by 

 increasing L2 . However, there is the practical limitation that increasing 

 L2 also increases the leakage inductance of transformer Ti , and in turn, 



I 



I 



-4.5V 



5.1K 



250A 



:iD2 



>3K 



A-l- 



OUTPUT 

 PULSE 



V- 



INPUT 

 WAVEFORM 



PULSE 

 AMPLITUDE^, 

 ADJUSTMENT^ 



•^ 



2.5 MEG POT. 

 I- 



jr 



ee' 



Ij, = 4 MILS 



L, = L2= 5 MILLIHENRIES 



L', = L2= 5 MILLIHENRIES 



COEFFICIENT OF 



COUPLING = 0.99 



REFERENCE 

 g^ LEVEL 



''adjustment 



MA 1 



I 

 I 



-46V 



I 

 I 



-t-1.5V 



100 OHM 

 POT. 



I 

 I 



-1.5V 



Fig. 21 — Voltage comparator. 



