594 BELL SYSTEM TECHNICAL JOURNAL 



engineering, particularly in communication engineering. One might almost 

 say that a problem is not solved unless an equivalent circuit has been found 

 whose performance will exhibit some of the characteristics of the actual 

 problem. This need for circuit concepts reflects a desire to make the 

 phenomena more alive and subject to physical interpretation, for it is true 

 that equivalent circuit concepts have greatly contributed towards physical 

 interpretation of analytical expressions. A problem may very well be 

 studied without recourse to the equivalent circuit concept and a correct 

 answer obtained, yet the development of an equivalent circuit adds greatly 

 to the complete interpretation of the physical phenomena. 



In this paper we shall be concerned only with linear a-c amplifier operation 

 where the term linear indicates that the analytical expressions connecting 

 currents and voltages are linear, that is, involve only the first power of any 

 instantaneous current or its derivative. We shall further restrict our atten- 

 tion to the usual mode of four-terminal amplifier operation in which one pair 

 of terminals is associated with the signal to be amplified and the other pair 

 with the amplified signal. The equivalent a-c circuit of such a transducer 

 will require the development and interpretation of the linear relations 

 connecting the a-c currents and voltages at the input terminals with corre- 

 sponding quantities at the output terminals. At this point we can logically 

 postulate that the important formal difference between an active and a 

 passive four-pole lies in that the law of reciprocity no longer can be assumed 

 to hold for the active network. Since passive four-poles require three inde- 

 pendent parameters for their complete specification the active four-pole 

 will require at least one additional parameter. 



In the practical applications we shall be principally concerned with the 

 various triode four-pole connections. A short review of the various stages 

 involved in deriving the newer forms of equivalent triode circuits will there- 

 fore be considered prior to the introduction of generalized concepts. Such 

 a review is in the main concerned with the effect of frequency upon the 

 early forms of the equivalent triode circuit. 



In the review we shall confine ourselves to the usual grounded cathode 

 mode of operation since it is only in recent times that grounded grid and 

 grounded plate operation have come into use. This distinction is, however, 

 not necessary and is introduced solely for simplicity- 



The conventional notation as well as the positive current directions are 

 indicated on Fig. 1 for a general four-pole N. It is assumed here that termi- 

 nals 1 are the available input and terminals 2 the available output terminals. 

 This choice of current direction appears to the writer to be the most con- 

 venient to use when the four-pole is energized at the input terminals 1 only. 



Consider, now, a grounded triode operated at such a low frequency that 



