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BELL SYSTEM TECHNICAL JOURNAL 



23. Feed-Back Amplifiers. This amplifier may be either resistance 

 or inductive coupled, a typical resistance coupled circuit being shown 

 in Fig. 31. In a feed-back circuit, attention must be paid to phase 

 relations. In Fig. 31, let the arrow along the resistance Ri represent 

 an increase in electron current to the grid of tube A. This corre- 

 sponds to an increase in the potential of this grid. In phase with this 

 increase in potential is an increase in electron current in i? 2 as shown 

 by the arrow. This, in turn, corresponds to a fall in potential of the 

 grid of tube B and therefore to a reduction of the I P in B, as indicated 

 by the arrow at R s , which produces an increase in I p in C. There- 

 fore, in this particular circuit the correct phase relations require the 

 output of one tube to be returned to the input of the second preceding 



Fig. 31 



tube or one of its alternate preceding tubes. The amount of energy 

 fed back can readily be controlled by varying the portion of Ro through 

 which the feed-back current flows. 



24. Push-Pull Amplifier. See Fig. 32. This type of circuit is 

 particularly useful as a terminating stage since it makes possible the 

 use of a low impedance in the output circuit without serious distortion. 

 If the tubes A and B have identical characteristics, it is readily seen 

 that the coils of the output transformer may be so connected that the 

 fundamental and odd harmonics will aid one another, while all even 

 harmonics will oppose. Since the third and higher harmonics (count- 

 ing the fundamental as first) are very small compared to the second, 

 this circuit gives very nearly distortionless amplification. In speech 

 amplifiers it permits of considerable overloading without this being 

 very apparent in the quality of the output. 



By reversing the transformer connections it is possible to cause 

 the circuit to add the even harmonics and give the differences of the 

 odd. 



An additional use for this circuit will be pointed out in the section 

 dealing with modulation. 



Fig. 33 shows a special type of push-pull circuit which is particu- 

 larly adapted to the amplification of steady and low frequency volt- 



