302 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



2.2. Methods Used to Shape the Loop Current Transmission 



An essential consideration in the design of a feedback amplifier is the 

 provision of adequate margins against instability. In order to accomplish 

 this objective, it is necessary to choose a criterion of stability. In Ap- 

 pendix I it is shown that it is convenient and valid to base the stability 

 of single loop transistor feedback amplifiers on the loop current trans- 

 mission. In order to calculate the loop current transmission of the dc 

 amplifier, the feedback loop is opened at a convenient point in the cir- 

 cuit, usually at the base of one of the transistors, and a unit current is 

 injected into the base (refer to Fig. 24). The other side of the opened 

 loop is connected to ground through a resistance (r^ -j- r^) and voltage 

 Veli • In many instances, the voltage re/4 can be neglected. If | Zj? | and 



3=1 Zj 



I 



are much greater than | Z 



IN 



then A/3 is very nearly equal to the loop 

 ciu'rent transmission. For absolute stability^ the amplitude of the loop 

 current transmission must be less than unity before the phase shift 

 (from the low frequency value) exceeds 180°. Consequently, this charac- 

 teristic must be controlled or properly shaped over a wide frequency 



10 



_J 



LU 



O 

 LU 

 Q 



< 



o 



\- 

 z 



LU 



a. 

 o 



•180 



-200 



-220 



•240 



•260 uj 



_l 



2 

 < 



-280 



•300 



LU 



< I 



I 



Q- 



-320 



■340 



,02 2 5 ,q3 2 ^ ,o4 •=; S ,q5 t S ,q6 



5 ■/^4 2 5 ,„s 2 5 ,„6 2 5 ^q7 2 5 jq8 

 FREQUENCY IN CYCLES PER SECOND 



Fig. 5 — Current transmission of a common emitter stage. 



