1222 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



turning points the approximation is by no means accurate although 

 affording zero order information. 



Preparatory to the analysis of the negative resistance characteristics, 

 it is necessary to obtain analytic expressions for the transistor currents 

 and voltages. This in turn involves the following steps: 



1. Identification of the three regions in terms of the device character- 

 istics, 



2. Idealization of the device characteristics to obtains simple, linear 

 relations, and 



3. E\'aluation of the device parameters in each of the three regions. 

 Fig. 13 is a family of open circuit characteristics for a typical switching 



type transistor. Specifically, in small signal terms, 



Table I 



Parameter 



Rn = 



Rl2 = 



Rn = 



R22 == 



die 



dVc 

 dL 



Also 



die 



dL 



R21 



R22 



Equivalent Tee 

 Rn = r, + Ti 



Ru = n 



R21 = Tm + n 



R22 = Tc + n 



To + n 



The above set, normally employed for small signal analj^sis, will be 

 assumed to be constant within a given region, but changing in value 

 from region to region. 



IDENTIFICATION OF THE THREE REGIONS 



It may be recalled with the aid of Fig. 12 that the negative resistance 

 characteristic consists of a negative resistance region bounded on each 

 side by a region of positive resistance. Thus the device is first passive 

 in nature with little or no gain, then very abruptly exhibits considerable 

 gain with the resultant negative resistance, and finally becomes very 

 abruptly passive again with little or no gain. 



It would seem quite clear that abrupt changes in the transmission 

 properties of a device should be associated with equally abrupt changes 



