2 BELL SYSTEM TECHNICAL JOURNAL 



as to throw away the excess gain, it has been found possible to effect 

 extraordinary improvement in constancy of amplification and freedom 

 from non-linearity. By employing this feedback principle, amplifiers 

 have been built and used whose gain varied less than 0.01 db with a 

 change in plate voltage from 240 to 260 volts and whose modulation 

 products were 75 db below the signal output at full load. For an 

 amplifier of convc ntional design and comparable size this change in 

 plate voltage would have produced about 0.7 db variation while the 

 modulation products would have been only 35 db down; in other 

 words, 40 db reduction in modulation products was effected. (On an 

 energy basis the reduction was 10,000 fold.) 



Stabilized feedback possesses other advantages including reduced 

 delay and delay distortion, reduced noise disturbance from the power 

 supply circuits and various other features best appreciated by practical 

 designers of amplifiers. 



It is far from a simple proposition to employ feedback in this way 

 because of the very special control required of phase shifts in the 

 amplifier and feedback circuits, not only throughout the useful fre- 

 quency band but also for a wide range of frequencies above and below 

 this band. Unless these relations are maintained, singing will occur, 

 usually at frequencies outside the useful range. Once having achieved 

 a design, however, in which proper phase relations are secured, expe- 

 rience has demonstrated that the performance obtained is perfectly 

 reliable. 



Circuit Arrangement 



In the amplifier of Fig. 1, a portion of the output is returned to the 

 input to produce feedback action. The upper branch, called the 

 /x-circuit, is represented as containing active elements such as an 

 amplifier while the lower branch, called the j8-circuit, is shown as a 

 passive network. The way a voltage is modified after once traversing 

 each circuit is denoted /x and ^ respectively and the product, ^i/3, repre- 

 sents how a voltage is modified after making a single journey around 

 amplifier and feedback circuits. Both /x and j8 are complex quantities, 

 functions of frequency, and in the generalized concept either or both 

 may be greater or less in absolute value than unity.^ 



Figure 2 shows an arrangement convenient for some purposes where, 

 by using balanced bridges in input and output circuits, interaction 

 between input and output is avoided and feedback action and amplifier 

 impedances are made independent of the properties of circuits con- 

 nected to the amplifier. 



* /x is not used in the sense that it is sometimes used, namely, to denote the 

 amplification constant of a particular tube, but as the complex ratio of the output 

 to the input voltage of the amplifier circuit. 



J 



