200 BELL SYSTEM TECHNICAL JOURNAL 



is connected to the middle of the coil, the grid to one end and the plate 

 is connected through a small balancing condenser to the other end 

 of the coil. In Fig. 13 (b) is given a schematic diagram of the circuit, 

 which shows that the effect of Cj, upon the coil circuit is just opposite 

 the effect of Cg-p, so that the circuit can be regarded as an ordinary 

 bridge circuit. It will, therefore, always be possible by proper ad- 

 justment of the condenser Cb to neutralize the effect of the feed-back 

 action as shown by curve G in Fig. 12. 



The same kind of an arrangement can be used between the different 

 stages in a multi-stage high frequency amplifier, and it is thus seen 

 that by proper use of such balancing condensers, it will be possible 

 to obtain for a multi-stage amplifier a total amplification which is 

 practically equal to the product of the amplifications per stage. This 

 is true for a multi-stage tuned circuit coupled amplifier but for trans- 

 former coupled amplifiers, where it is more difficult to obtain a 180° 

 phase difference of voltages, the advantage of the balancing con- 

 denser is not so great. 



Of course, this favorable result [)rcsupposes that the wiring of tJie 

 amplifier is properly done and the different stages shielded carefiilh' 

 from each other so that no external coupling exists between them. 



What has been said about amplifiers in the preceding sections can 

 be summarized as follows: 



With a given type of amplifier the same general shape of the am- 

 plification curve is obtained regardless of the fi(.(|iK ika- range at 

 which the amplifier is designed to operate. 



Thus, a low amplification over a wide frequency range will be 

 obtained by using loosely coupled transformers or choke coils without 

 any step-up, while a high amplification o\'er a narrow range of fre- 

 quencies can be obtained by using choke coils or tuned circuits with a 

 proper steji-up. In this last case, it will be necessar>' to use a small 

 tuning condenser across the coils in order to make the frequency 

 range of the amjilifier wider, and the higher amount of amplifica- 

 tion is, therefore, obtained only by a sacrifice of tuning facilities of 

 the set. In a multi-stage amplifier it may, however, often be foimd of 

 advantage to use a combination of low amplification stages and high 

 amplification stages so that, for instance, one tuned circuit stage with 

 high steji-up and variable condenser is used in connection with one 

 or several stages of choke coils or loosely coupled transformers with 

 low amplification and a wide frequency range. The maximum amplifi- 



f ' 



