THEORY OF NEGATIVE IMPEDANCE CONVERTER 



107 



current does not reverse direction the voltage must, to produce a negative 

 impedance. 



If Equation (14) for the current U were studied it would be found that 

 all conditions which meet the requirement for circuit stability when the 

 impedance Z34 is negative would produce a flow of current through this 

 mesh with a phase impossible to realize were Z34 positive. Hence, if im- 

 pedance Z34 is to be negative the current must be reversed and the im- 

 pedance must be of the reversed current type equal to V/—I, where here 

 V is the voltage across Z34 and / is the current flowing through it. 



Design Considerations 



If a resistance were connected to the network terminals 3 and 4 of Fig. 

 6(a) the impedance seen at the line terminals 1 and 2 would follow a locus 



Fig. 11 — Impedance locus of El repeater. 



similar to that shown in Fig. 11 for the frequency range from zero to in- 

 finity. At zero frequency this impedance would be a small positive resistance 

 equal to the d-c resistance of the primary windings of transformer T. 

 At some low frequency /i (Fig. 11) the locus would show a positive im- 

 pedance. In the El repeater it is this portion of the impedance characteristic 

 which is used for the passage of low frequencies such as ringing, diahng 

 and the like. Between the frequencies of /2 and/3 (Fig. 11) is seen an im- 

 pedance which approximates a negative resistance. At high frequencies the 

 locus would approach the origin again. In Fig. 11 this approach is shown 

 through the first and then the fourth quadrant. At frequencies above the 

 speech band passed by the telephone line a negative impedance is not 

 wanted for the El repeater because it is of little value for voice transmission 

 and may be detrimental in adding to the difficulty of obtaining stable 

 operation. 



