TRANSISTORS IN SWITCHING CIRCUITS 



1217 



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5 -If 



TRIGGER 



Tt 



TRIGGER 



Fig. 8 — Idealized monostable relaxation oscillator waveforms. 



When the load line and bias are chosen to result in intersection in the 

 negative resistance portion, astable operation or continuous oscillation 

 results. This mode is illustrated in Fig. 7(c). Proper bias and Rt > \ —Rin \, 

 Region II, are required. The operating point formed by the intersection 

 of the load line on the negative resistance portion of the characteristic 

 would normally be stable. However, the capacitor provides an ac short- 

 circuit in parallel with R^ causing the path (1), (2), (3), (4) to be followed 

 continuously. Another form of physical explanation of this relaxation 

 oscillation, usually applied to gas tubes, is that the capacitor C is 

 charged slowly throuch Rt to a critical or breakdown value whereupon 

 the tube or device rapidly discharges the capacitor. When the capacitor 

 charge is dissipated, the device discharge can no longer be maintained 

 due to the IR drop in Rt and the tube or device open-circuits and the 

 capacitor is recharged. 



The above suggests a strong simularity to gas tube behavior and this 

 is indeed so. In fact, the modes described above are common to all 

 open-circuit stable negative resistance devices; only the parameters 

 and device phenomena are different. 



The primitive circuits of Fig. 7 have pi'operties basic to several 

 switching functions. These may be deduced from the waveforms of Fig. 

 8 which are essentially identical to both the mono.stable and astable 

 cases. The emitter current has a rectangular Avaveform which suggests 

 the generation of rectangular pulses; and, for the astable case, regenera- 

 tive amplification for both amplitude and wave shape, pulse rate or 



