TUANSISTOUS AND JUNCTION DIODES 



839 



\\hich should not be exceeded under any operating conditions, and this 

 must be kejit in mind during the design of the reguhiting circuits. 



2.35. Equivalent Circuit of a T7'ansistor 



Ryder and Kircher^ have shown that it is possible to convert the 

 sketches shown in Fig. 7 into a small signal etiuix'alent circuit using 

 alpha and the three characteristic resistances of the transistor. These 

 resistances are the emitter resistance Vc , the base resistance fh and the 

 collector resistance i\. . Two forms of equivalent circuit are shown in Figs. 

 9(a) and 9(b). In the equivalent circuit in P'ig. 9(a) the active portion 

 of the transistor is characterized as a current generator. This cciuivalent 

 circuit is more directly related to the physical piocesses occurring inside 

 the transistor than the eciuivalent circuit in Fig. 9(b) which characterizes 

 the active portion of the transistor as a voltage generator. Although both 

 equivalent circuits are useful the one in Fig. 9(a) is preferred in power 

 work because t'c is much larger than the load resistance in many cases 

 and can be neglected. Typical ^'alues for the eciuivalent circuit param- 

 eters are given in the caption of Fig. 9. The use of the equivalent circuits 

 are further discussed in some of the articles listed at the end of this paper. 

 The article^ by R. L. Wallace Jr. and W. J. Pietenpol is of particular 

 interest in this connection. 



2.36. Typical Junction Transistors 



Fig. 10 is a photograph of two Bell System junction transistors made 

 from germanium. The smaller one will dissipate 50 milliwatts, and the 

 larger one is an exploratory model that will dissipate 2 watts when it is 

 attached to a suitable heat sink. These transistors are hermetically 

 sealed to protect them from the infiltration of moisture. The characteris- 

 tics shown in Fig. 8 were measured using the smaller unit. 



Veb 



i 

 T o 



-'/.A- 



l = aU 





(a) 



Vcb 



i 



t 



Veb 



1 



1^ 



c \AAr 



,4 s 



i 



v=rml 



+_ 

 arc) 



1 



Vcb 

 \ 



-O T 



fb) 



Fig. 9 — Junction transistor equivalent circuits. Typical values for a .SO-mil- 

 liwatt transistor: /•„ , 25 ohms; n , 500 ohms; ;> , 5 megohms; a, 0.98; and /„, , 4.9 

 megohms. 



