330 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



Te+rb 



^6^4 ( 'V 



-aZcLb 



N 



COMPLETE 

 AMPLIFIER 

 EXCLUSIVE 



OF THE 

 TRANSISTOR 

 IN QUESTION 



Fig. 24 — Measurement of loop current transmission. 



ground through a resistance (/•<; + n) and a voltage r J4 • Using the 

 nomenclature given in Reference 8, the input of the complete circuit is 

 designated as the first mesh and the output of the complete circuit is 

 designated as the second mesh. The input and output meshes of the 

 transistor under consideration are designated 3 and 4, respectively. The 

 loop current transmission is equal to I3', the total returned current when 

 a unit input current is applied to the base of the transistor. 



The return difference for reference Ve is equal to the algebraic differ- 

 ence* between the unit input current and the returned current h'. 1 3 is 

 evaluated by multiplying the open circuit voltage in mesh 4 (produced 

 by the unit base current) by the backward transmission from mesh 4 to 

 mesh 3 with zero forward transmission through the transistor under 

 consideration. The open circuit voltage in mesh 4 is equal to (re — aZc). 

 The backward transmission is determined with the element aZc , in the 

 fourth row, third column of the circuit determinant, set equal to Ve . 

 Hence, the return difference is expressed as 



A43 



Fr' = 1 + {aZc - re) 



(Al)t 



Fr' = 



A''* + {aZc - r.)A 



43 



ir', 



(A2) 



Fr'.= 



A^'' 



= 1+ Tr' 



(A3) 



The relative return ratio Tr', is equal to the negative of the loop current 

 transmission and can be measured as shown in Fig. 24. The voltage reh 

 takes into account the fact that the junction transistor is not perfectly 



* The positive direction for the returned current is chosen so that if the original 

 circuit is restored, the returned current flows in the same direction as the input 

 current. 



t A''« is the network determinant with the element aZc in the fourth row, third 

 column of the circuit determinant set equal to r, . 



