602 



BELL SYSTEM TECHNICAL JOURNAL 



We need, to begin with, the well-known network representation of a 

 passive four-pole. Equation (14) has, then, the form 



/i = iSiiFi + ^nV2 1 



(15) 



h = -/3l2Fl + ^22F2J 



and one equivalent circuit representation is that given by the 11 network 

 having the element values shown in Fig. 7. If so desired the 11 network 

 can of course also be transferred into an equivalent T network. 



-y3t2 



Fig. 7 — Equivalent circuit of a passive four-pole. 



->3l2 (/3l2+/320V,| 



1^- 



/3n +>9l2 -{/^22'y^\z) 



l^' 



Fig. 8 — Equivalent circuit of an active four-pole; current impressed at the output. 



Now write (14) as 



h = iSiiFi + )8i2F2 ] 



h = -^i2Fi -f J822F2 + (^12 + 1821) Fi J 



(16) 



Whence it is seen by a comparison with (15) that the network representation 

 of the active four-pole differs from the passive one merely by the presence 

 of the impressed current 0(3i2 r 1821) Vi . A possible network representation 

 of the general active four-pole is thus as shown on Fig. 8. 



An immediate application may be illuminating. Consider, for example, 

 the triode operated with positive grid, with interelectrode capacitances 

 taken into account. The four-pole equations are given by (11) and (12) 

 and the classical network is that of Fig. 5. From (12) and Fig. 8 we get the 

 network of Fig. 9. It may be observed that, while in Fig. 5 the source and 

 source-free constituents are intermingled, this is not the case in Fig. 9 

 where, on the contrary, a clear demarcation is present between such con- 

 stituents. 



