i 



THEORY OF NEGATIVE IMPEDANCE CONVERTER 101 



quent reduction in gain. Under excessive overload conditions the negative 

 impedance becomes small; and the effect of the converter is scarcely dis- 

 cernible on the transmission of speech as far as gain is concerned. The 

 harmonic distortion introduced on the line by the vacuum tubes overloading 

 will increase to a maximum and then decrease with further increase in 

 volume. The constants of the circuit coupling the plate to the grid of the 

 vacuum tube or tubes in the converter will determine the maximum amount 

 of distortion. A push-pull circuit is better from this standpoint than a 

 single sided one. In the El repeater harmonics are not particularly ob- 

 jectionable under any condition of overload. 



The El repeater employs a single 407 A vacuum tube which is a twin 

 triode of the 9-pin miniature type. When operated on a plate voltage of 

 130 volts this repeater will pass speech volumes of +10 vu before com- 

 pression begins because of overloading in the vacuum tube. 



Development of the El Equivalent Circuit 



One purpose of this paper is to prove that Fig. 6(b) is equivalent to 

 Fig. 6(a) 



Assume that an impedance Zn is connected across terminals 3 and 4. 

 Assume, furthermore, that the elements Ci, C2, i?3 and R^, which also 

 connect across terminals 3 and 4, are all included in impedance Zn. Assume 

 an impedance Zl is connected to terminals 1 and 2. The circuit of Fig. 6(a) 

 then can be represented by Fig. 8 where the vacuum tubes have been 

 replaced by their plate resistances (Rp) and their equivalent circuit voltages 

 Miei and /X2^2- The voltage /xi^i is that voltage which appears in the plate 

 circuit of each tube as a result of the action on the grid of all voltages 

 between the tube plates and ground. The voltage ^2^2 is that which appears 

 in the plate circuit of each tube as a result of the voltages between the 

 cathode and ground. Fig. 6(a). The resistors Ri, R2 and Rg designate the 

 resistance in the various coil windings plus other circuit resistance which 

 might be inserted at the points indicated. The reactances Xi, X2 and X3 

 represent the effect of the self -inductance in the coil windings. The react- 

 ances Ml, M2 and M^ represent the effect of the mutual inductances between 

 coil windings. The numbers on a coil winding determine the polarity of the 

 winding with respect to other windings on the same core. As mentioned 

 previously the vacuum tubes are operated Class A. 



The basic circuit equations can be written as follows for Fig. 8: 



= P7i + Qh - M3(l + H2)h - S(l - ni)h 



= e/i -f PI2 - M,{1 + M2)/3 - 5(1 - /Xl)/4 



£3 = -M3/1 - if 3/2 + {Zl + R, + X^)h + 

 £4 = -Sh - 5/2 + + (Z^ + 25)/4 



