TRANSISTORS IN' SWITCHING CIRCUITS 



1211 



signal is below (he initial (hi-eshoUl there is no response and any change 

 is directly related to the passive transmission of the control signal alone. 

 When the signal exceeds the threshold the second state is assnmed. 

 Watch escapements, thyratrons, antl the whole family of oscillators 

 fall into this category. When the simplest cases of such functions are 

 analyzed, they are found to involve in one way or another two stable 

 states separated by a region in which there is positi\'e feedback and 

 gain in excess of unity with a resultant equivalent negative resistance. 

 The proposition that a negative resistance characteristic is common to 

 trigger or threshokl switching circuits is tacitly assumed. The next step 

 is to examine tlu^ transistor for such behavior and to classify the proper- 

 ties. 



NEGATIVE RESISTANCE IN THE TRANSISTOR 



That the transistor* can exhibit negative resistance has been demon- 

 strated analytically^ and experimentally. The resistances seen looking 

 into the emitter and collector of the transistor with grounded base are 

 shown in Fig. 2. 



In the equations and discussion to follow, the symbol conventions 

 are as follows: External circuit elements are capitalized as R^ , Rb , and 

 Re . The symbols Rn , R12 , Rn and R21 define the open-circuit transistor 



Vm , and Vb define the equivalent circuit 



resistances; the svmbols r^ , Vc 



R|N 



[^ 





RiN — R? 



f^iz '^21 



Tc+rb- 



rb(''b+ I'm) 



R'„ ■'■ ■" Te+rb+Rf 



Fig. 2 — Emitter and collector driving point resistances. 



* Discussion is limited primarily to point contact transistors with a's or cur- 

 rent gains greater than unity. 



* R. M. Ryder and R. J. Kircher, "Some Circuit Aspects of the Transistor", 

 Bell System tech. J., 28, pp. 367-400, July, 1949. 



