SOME CIKCL ir AS/'J-X TS ()/■ THE fKANSLSTUK 379 



The grounded emitter connection can also exhibit bilateral properties. 



Recapitulating these three single-stage amplihers, we see that when a = 1 

 their properties are close enough to the analogous electron tube arrange- 

 ments to be easily remembered; but that, when a is different from 1, their 

 properties begin to diverge from their tube counterparts. Some of these 

 circuits will perform in a simple manner functions which are impossible to 

 the analogous tube connections, although of course the functions could be 

 accomplished by using more tubes or more complicated circuits. 



Frequency Response 



So far the analysis of transistors has been given only for the resistive 

 case, appropriate at low frequencies. When the frequency is raised, reactive 

 components appear and the situation becomes more complicated, although 

 of course still subject to the same general methods of analysis. 



One might expect that smce semiconductmg diodes work at microwave 

 frequencies, so also would semiconducting triodes. For the Type A transistor, 

 this hope is blasted because of the essentially different nature of the mecha- 

 nism, mvolving as it does the physical transport of charge carriers over ap- 

 preciable distances. For certain features of the transistor, however, the 

 analogy does hold. For example, the emitter by itself is a diode; and, in 

 keeping with this fact, its open-circuit impedance does not change much 

 with frequency in the range in which we shall be interested. For most en- 

 gineering purposes the open-circuit input impedance of a Type A transistor 

 may be regarded as a resistance independent of frequency. Such deviations 

 as occur are small and entirely similar to what take place in an analogous 

 diode. 



The same situation holds with respect to the base resistance rb and the 

 collector resistance re, that is, they act as one might expect of a diode. The 

 base resistance is substantially constant with frequency; the collector re- 

 sistance has associated with it a slight amount of capacitance, mostly due 

 to the case, leads, and wiring external to the unit, which gives a variation 

 of properties with frequency in high-impedance circuits. The analogous 

 capacitance on the emitter side is negligible because of the lower value of 

 emitter impedance. One has, therefore, the T of resistors in the equivalent 

 circuit substantially constant with frequency. 



The dominant factor governing frequency response of the transistor is 

 therefore largely expressed as a variation of the net mutual impedance r.u or, 

 one may say as well, in the factor a which is the ratio of rm to ro. 



Measurements of r^ as a function of frequency encounter the practical 

 difficulty that it is impossible to present to the transistor over a wide fre- 

 quency range an impedance high compared to the collector impedance. It 

 is, however, quite easy to present to the collector a relatively low impedance 



