14 



BELL SYSTEM TECHNICAL JOURNAL 



shows measurements on a different amplifier in which harmonics are 

 reduced as negative feedback is increased, db for db over a 65 db range. 

 That the gain with frequency is practically independent of small vari- 

 ations in I /x I is shown by Fig. 9. This is a characteristic of the Morris- 

 town amplifier described in the paper by Messrs. Clark and Kendall ^ 

 which meets the severe requirements imposed upon a repeater amplifier 

 for use in cable carrier systems. Designed to amplify frequencies from 4 kc 



95 

 90 

 85 

 80 

 75 

 70 

 65 

 60 

 55 

 50 

 45 

 40 

 35 

 30 

 25 

 20 



70 < 



5 10 15 20 25 30 35 40 45 50 55 60 65 

 DB REDUCTION IN GAIN DUE TO FEEDBACK 



Fig. 8 — Improvement of harmonics with feedback. One example of another 

 amplifier in which with 60 db feedback, harmonic currents in the output are only 

 one-thousandth and their energy one-millionth of the values without feedback. 



to 40 kc the maximum change in gain due to variations in plate voltage 

 does not exceed 7/10000 db per volt and at 20 kc the change is only 

 1/20000 db per volt. This illustrates that for small changes in |/t|, 

 the ratio of the stability without feedback to the stability with feed- 

 back, called the stability index, approaches 1 1 — /i|3| V(l ~ |i"|S| cos $) 

 and gain stability is improved at least as much as the gain is reduced 

 and usually more and is theoreticaljy perfect if cos <l> = 1/|m/3|. 



^Loc. cit. 



