1218 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



frequency division and delay. As shown the current waveform is not 

 particularly good, having neither a flat top nor a fiat base line. Practically, 

 the waveform may be derived from the collector by means of a small 

 load resistor to obtain a flat base line. When the emitter current is 

 negative there is sensibly no transfer action, hence, the collector current 

 will be constant during the re-charge portion of the cycle instead of 

 exponential as shown. The slope of the top is inherent and may be 

 removed by clipping. Pulse rise time, the time required for transition 

 from low current to high current, of 0.1 us is quite easily obtainable; 

 0.02 us with average input powers of 20 7nw have been obtained. Fall 

 time is usually longer than the rise time by factors of 3 or 4. It is to be 

 noted in Fig. 8 that there has been shown a delay between the trigger 

 application and the current transition. Such delay is not peculiar to 

 transistors, but is common to all trigger type devices and circuits. The 

 delay is shown here exaggerated in order to establish its existance and 

 is associated with the static charging of the circuit and the d3aiamic 

 delay of the de\dce concerned. The trigger-transition delay with tran- 

 sistors is usually less than 0.1 us. 



The voltage waveform of Fig. 8 has a sawtooth form and may thus 

 be employed to generate linear time bases or sweeps. The normal 

 methods for linearization such as a high charging voltage F„ and a 

 high charging resistance Re or other constant current means are appli- 

 cable here as in other device circuitry. Free-running and driven sweeps 

 may be obtained with the astable and monostable circuits respectively. 



Since the collector characteristic sho^^^l in Fig. 3 is also open-circuit 

 stable, the same sort of circuits can be constructed using the output 

 characteristic. Bistable, monostable and astable circuits are shown in 

 Fig. 9. 



The resistances seen looking into the base are given in Fig. 10. These 

 circuits are short-circuit stable. That is, high values of Rb result in 

 instability. Bistable, monostable and astable circuits can be constructed 

 also, but use is made of an inductor instead of a capacitor. The reactance 

 of the inductor affords a quasi-open-circuit in the same manner as the 

 capacitor afforded a cjuasi-short-circuit in the previous cases. Circuit 

 examples are shown in Fig. 11. 



SUMMARY 



These simple circuits by no means exhaust the s\vitching circuit 

 possibilities of the transistor; rather, they are the simplest. The simple 

 circuit is often satisfactory and may sometimes be employed with little 

 more understanding than that given. More often, however, problems 



