SOME ClRCLir ASPECTS 01' THE TR.l.XS/STOR 3S9 



The foregoing amplifiers both have rather low output powers because of 

 the fact tliat the emitter, a low-current electrode, is the output electrode. 

 A wav of improving this situation has been suggested in the second amplifier 

 schematic shown in Fig. 22. The first stage is a grounded emitter and the 

 second a grounded collector transistor, the latter operating in what we have 

 called the "backward" direction so that the output electrode is the base and 

 the power level is improved. This amplifier can be stabilized by negative 

 feedback obtainable by inserting a resistor in the first stage emitter lead. 



These e.xamples emphasize that one can cascade unlike stages and that 

 feedback can be used to stabilize performance, just as with electron tubes. 

 These amplifiers can be further cascaded to obtain more gain. Other pos- 

 sibilities worthy of mention include modifying the design of the first stage 



(a) (b) 



Fig. 22 — Non-iterative cascade amplifiers. 



of an iterative amplifier to obtain good noise figure, or of the last stage 

 for greater power output. 



Band P.ass Amplifiers 



Bandpass amplifiers require a few remarks before concluding the small- 

 signal discussion. The design within the band may be carried out by the 

 methods previously discussed; but frequently attention must also be paid 

 to properties outside the band, to an extent unusual with tubes. The reason, 

 of course, is connected with that Dr. Jekyll and Mr. Hyde of transistors, 

 a (or a) greater than 1. Wlien a transistor may be short-circuit unstable, 

 then oscillations may result from the practise usual with electron tube 

 amplifiers of letting the impedances outside the band fall to low values. For 

 the same reason design of power leads requires more care than usual. The 

 problems encountered are somewhat similar to those of tube amplifiers 

 with feedback in that one must pay attention to characteristics far outside 

 the useful band. In the case of transistors, one may have to exercise design 

 care to avoid oscillations even when the gain of the amplifier is less than 

 unity. 



Large Signal Analysis 



Large signals are those which involve considerable excursions over the 

 electrical characteristics of the device and cannot be regarded as small 



