TRANSISTORS IX SAVITCHIXG CIRCUITS 



1215 



With the addition of suitable steering circuits it can be made to count 

 b}^ a scale of two. 



MONOSTABLE AND ASTABLE CIRCUITS 



The addition of a capacitor to the circuit as in Fig. 7(a) leads to 

 either nionostable or astable operation. In Fig. 7(b) the normal operating 

 point is stable at a as discussed previously b}^ virtue of the l)ias \'\( . 

 As Vft is increased, as b}" a trigger, the load line is mo\'ed up and over 

 tiie turning point. AVithout capacitor C in the circuit, the operating 

 point would move to b with the resultant rapid change in voltage and 

 current. However, a capacitor has in effect voltage inertia; this is 

 equi\-alent to saying that a capacitor is a short-circuit to a voltage 

 change. Both the capacitance and the rate of change of voltage are 

 assumed high. Thus at the turning point the capacitor effecti\'ely 

 short-circuits the emitter and the operating point snaps along dotted 

 line (1) to intersect the characteristic. This point is ciuasi-stable and 

 tiie capacitor is discharged along line (2) to the second turning point 

 where the emitter is again effectively short-circuited and the operating 

 point snaps along (3) to intersect the Region I portion of the character- 

 istic. This point is also quasi-stable and the operating point moves 

 slowly up to the initial or dc stable operating point. A single trigger 

 thus causes a complete cycle of operation. The emitter current shifts 



Fig. 6 — Bistable circuit showing trigger sensitivity, A. 



