THE TRANSISTOR AS A XIOTWORK KLEMENT 



331 



For convenience the simple forms of negative resistance may be di- 

 \ided into two general classes which are duals in a network sense. Since 

 those classes have been identified in the literature in several different 

 ways, it seems desirable to sunnnarizc the major characteristics in the 

 form given in Table I. 



'riiereforc a shunt negative resistance is one whose magnitude is 

 controlled mainly by the voltage across its terminals. It is short circuit 

 stable which means it must operate into a low impedance. When the 

 internal gain used to produce the effect is reduced, the magnitude of a 

 shunt negative resistance increases. In addition it should be associated 

 with a parallel capacitance to predict its behavior outside the working 

 hand of frec|uencies. 



One method of producing a two terminal shunt negative resistance is 

 to arrange a transistor as shown in Fig. 1(a). To facilitate prediction of 

 the behavior of this combination it is desirable to derive an equivalent 

 circuit. 



Equivalent Circuit of a Transistor Shunt Negative Resistance 



An equivalent circuit of the transistor and its associated network is 

 shown in Fig. 1(b). By denoting each condenser reactance as jX, the 

 circuit determinant, A, can be \\Titten as follows. 



Xext the input impedance is determined as A/An • 



From this formula the exact general expression for the input im- 

 pedance is found to be very cumbersome and will not be given. A us(^ful 

 appi'oximation can be found by making some simplifying assumptions 



(a) (b) (c). 



Fig. 1 — Equivalent circuit of transistor sfiunt negative resistance. 



