SOME CIRCLIT ASPECTS OE THE TluXSfSTUK 



381 



increment in emitter current at constant collector voltage.^ The quantities a 

 and a are usually nearly the same. 



An oscilloscopic presentation of a versus frequency is possible and is a 

 great convenience since many units can be measured quickly and variation 

 with operating point observed directly. The sweep frequency generator 

 built for this purpose is diagrammed in Fig. 12. It presents on an oscillo- 

 scope the magnitude of a as a function of frequency from to 15 megacycles. 

 Means are also available for making point-by-point plots which are more 

 accurate, though much slower. 



2 3 4 5 6 7 8 9 10 n 12 13 



CUT-OFF FREQUENCY, f^, IN MEGACYCLES PER SECOND 



Fig. 14 — Cut-off frequency statistics. 



A set of curves of current amplitication factor a versus frequency, as 

 obtained with this apparatus, is shown in Fig. 13. The cutofT shape is a little 

 sharper than that of a single R-C circuit but less so than that of a pair, one 

 of which is shunt-peaked enough to make the combination tlat. The ap- 

 parent high-frequency asymi)tote varies in different units from 7 to 11 db 

 per octave. 



The phase shift associated with this curve has been found to be related to 

 the amplitude in the same way as if the characteristic were that of a ''mini- 



» .\ctually, a = (c)Ic/3I<)vc is only one of a set of four circuit parameters h,j whose 

 relationship to I, and Vc is the same as that of the Z's to I^ and Ic, and which furnish an 

 alternative circuit representation of the transistor. The other three h's can Ije measured 

 in a similar manner but are of less interest. 



