1214 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



that the intersection of load hne Re with the characteristic at b in Fig. 5 

 is unstable whereas those at a and c are stable. Experiment in the multiple 

 intersection case shows also that as F„ is slowly increased (decreased in 

 absolute magnitude) the load line moves upward and that the assumed 

 operating point, a, moves up along the Region I portion of the charac- 

 teristic. At the turning point shown on the current axis, the operating 

 point suddenly flips to the high current region, returning along the 

 curve to c as F„ is returned to the original value. 



A decrease in V^e toward F^^ moves the operating point at c down- 

 ward along the characteristic until it "escapes" past the lower turning 

 point and flips to the Region I portion, returning to a as V^t is returned 

 to the original value. This then is an elementary switching circuit, a 

 bistable trigger circuit or "flip-flop". A positive emitter pulse will cause 

 the circuit to flip to high current, a negative pulse to low current. The 

 triggers may be applied to emitter, base or in combination with proper 

 attention to polarity. Trigger sensitivities are shown in Fig. 6. Such a 

 circuit is often used for register or storage purposes. It can store one bit 

 of information for a potentially infinite period, be sampled for the 

 presence of such information, and be cleared or restored to the original 

 condition for reuse when the stored information is no longer useful. 



Fig. 5 — Emitter negative resistance characteristic showing possible multiple 

 operating points. 



