. The Bell System technical journal . r while the lower branch, called the j8-circuit, is shown as apassive network. The way a voltage is modified after once traversingeach circuit is denoted /x and ^ respectively and the product, ^i/3, repre-sents how a voltage is modified after making a single journey aroundamplifier and feedback circuits. Both /x and j8 are complex quantities,functions of frequency, and in the generalized concept either or bothmay be greater or less in absolute value than unity.^ Figure 2 shows an arrangement convenient for some purposes where,by using balanced bridges in i
. The Bell System technical journal . r while the lower branch, called the j8-circuit, is shown as apassive network. The way a voltage is modified after once traversingeach circuit is denoted /x and ^ respectively and the product, ^i/3, repre-sents how a voltage is modified after making a single journey aroundamplifier and feedback circuits. Both /x and j8 are complex quantities,functions of frequency, and in the generalized concept either or bothmay 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, interactionbetween input and output is avoided and feedback action and amplifierimpedances 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 theamplification constant of a particular tube, but as the complex ratio of the outputto the input voltage of the amplifier circuit. J STABILIZED FEEDBACK AMPLIFIERS. ♦■E + N +D Fig. 1—Amplifier system with feedback. e—Signal input voltage. y.—Propagation of amplifier circuit. —Signal output voltage without feedback. n—Noise output voltage without feedback. d{E)—Distortion output voltage without feedback. /3—Propagation of feedback circuit. E—Signal output voltage with feedback. N—Noise output voltage with feedback. D—Distortion output voltage with feedback. The output voltage with feedback is E -\- N -\- D and is the sum of fxe -\- n -\- d{E),the value without feedback plus yu/3[£ + N + D] due to feedback. E + N + D=iJie + 7i + d{E) + n^lE + N + D^ IE + N + Z)](l - M/3) = fxe + n + d{E) E + N + D fie + + d{E) 1 - M/3 1 - M^ 1 - M/8 If |ju/3| ^ 1, £ = — -. Under this condition the amplification is independent of IX but does depend upon /3. Consequently the over-all characteristic will be con-trolled by the feedback circuit which may include equalizers or other correctivenetworks.
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Keywords: ., bookcentury1900, bookdecade1920, booksubjecttechnology, bookyear1
