300 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



equivalent circuit current) fed back to the input summing node. With 

 these definitions, 



he = A/in' (4) 



la - 131. 



sc 



where /sc is the Norton equivalent short circuit current. 

 From Kirchhoff's first law 



/in = /in + Iff 

 Combining relations (4) to (6) yields 



'sc 



A 



(5) 



(6) 



(7) 



/in 1 - A^ 

 Expression (7) provides a convenient method for evaluating the external 



Fig. 3 — Single loop feedback amplifier. 



voltage gain of an operational amplifier. Fig. 4 shows a generalized op- 

 erational amplifier with N inputs. With this configuration, 



IN 



j=i L 



TTT he r, I 



Zj 



(S) 



where Ej , j = 1,2, • ■ • , N, are the N input voltages referred to the 

 ground node. 

 Zj,j— 1,2, • ■ ■ , N, are the A^ input impedances 

 ZiN is the input impedance of the amplifier measured at the 

 summing node with the feedback loop opened. 



Eo 



//i 



sc 



UT 



'IN 



la = 



A 



Eovr = 



Zk 



/sc ~ / 



(3 



Rl Zovt 



(5>) 



(10) 



