3S4 BELL SYSTEM TECHNICAL JOCRXAL 



Other types of variations of cutoff frequency with collector voltage are 

 exhibited by some transistors. 



That frequency cutoff is affected by the spacing between points of the 

 transistor is shown in Fig. 16, which gives some support to the idea that 

 the cutofif frequency might vary inversely as point spacing, other things 

 being equal. However, one has only to look at the graph to see that other 

 things are not equal for, at any given point spacing, the cutofif frequencies 

 of different units vary by almost an order of magnitude. It is, however, clear 

 that point spacing is one of the important factors. 



In recapitulation of the measurements of frequency behavior, it appears 

 possible to build Type A transistors with frequency cutofifs well above 10 

 megacycles. At the present time, the factors determining the frequency 

 behavior are not yet under good control. 



Cascade Amplifiers 



Many cascading possibilities exist, since any connection of the transistor 

 might be used in combination with other connections, as well as involving 

 all the parameter variations which might be made on each single stage. 

 Some of the more elementary possibilities will be mentioned. Since feedback 

 in each unit greatly complicates the situation, the essential features of the 

 amplifiers may become clearer by discusing an idealized case where feedback 

 is absent or greatly reduced. For similar reasons, the preliminary discussion 

 is confined to frequencies low enough so that the equivalent circuits are 

 purely resistive. 



Perhaps the most straightforward cascade amplifier is the iterated 

 grounded-base cascade, outUned in Fig, 17. Neglecting feedback, the in- 

 sertion power gain is nearly equal to the current amplification factor a 

 squared. For the Type A transistor this amounts to some 5 db per stage. 

 For most uses this could be regarded as unpractically low, but it might be 

 pointed out that the tube analog (grounded grid cascade) is even worse; 

 for when a = 1 the maximum insertion gain is db per stage. Both am- 

 plifiers of course can be made practical by interstage transformers (Fig. 18). 

 For the Type A transistor, the matched gain without feedback rises to about 

 15 db per stage, which still compares favorably in magnitude with most 

 grounded-grid tubes. 



When feedback is considered by allowing rb to return to its usual value 

 of a few hundred ohms, the question of stability becomes important. The 

 nominal Type A transistor is still stable when the cascade interstages are 

 matched, the gain rising to about 21 db per stage. For many units having 

 more than the usual amount of feedback, the interstages cannot be matched 

 without violating the stability condition and therefore encountering os- 



