NEGATIVE IMPEDANCE TELEPHONE REPEATERS 1075 



the converter through terminals 1 and 2, terminals 3 and 4 would be 

 used. However, because the E3 must be designed for connection across 

 a transmission line a coil or transformer input is not practical since the 

 coil would shunt the line at low frequencies and introduce excessive loss 

 to dial pulsing and 20 cps ringing. Without a coil to step up the im- 

 pedance of the line, variations in 27?p/(l + n) with standard triodes are 

 too large to be neglected. For this reason another converter circuit was 

 designed for the E3 repeater. 



This circuit is shown in schematic form in Fig. 15(a). It consists of 

 two resistances, a and c, respectively, and an amplifier poled according 

 to the plus and minus designation on Fig. 15(a). The output impedance 

 of the amplifier has been designated as 6. If the input impedance of 

 this amplifier is high compared to other circuit impedances. Fig. 15(a) 

 can also be represented by the equivalent circuit of Fig. 15(b). Here is a 

 conversion factor similar to that in the E2 converter and also a series 

 impedance. The factor n is the ratio of the open circuit output voltage 

 to the input voltage of the amplifier. In the E3 converter this voltage 

 ratio n is quite high because the amplifier is a two-stage arrangement. 

 In the design of the E3 both 0i and ^2 are approximately one half. Thus 

 nl3i and m/32 are both large compared to unity so that the conversion ratio 

 (1 — M/3i)/(l + f^^i) is approximately unity and relatively independent 

 of variations in n. Furthermore, because ^(32 is large compared to 6, the 

 series term in the converter circuit is relatively small and variations in 

 this term have little effect on the operation of the converter. 



Circuit Description 



THE E2 telephone REPEATER 



The circuit function of the E2 telephone repeater can be divided into 

 two parts: the electron tube (negative impedance) converter; and the 

 adjustable two-terminal network associated with the converter. 



In order to reduce the effect of variations in the electron tubes to 

 negligible proportions, and at the same time to operate the tubes with 

 load impedances that will permit optimum energy transfer from tube to 

 connected circuit, the impedance of the telephone line is stepped up 

 by means of the input transformer. To insure adeciuate balance for use 

 in the telephone lines, the low voltage side of the transformer is divided 

 into two equal, balanced windings. Each winding is center-tapped and 

 connected in series with a fine conductor. The circuit of the E2 repeater 

 is shown in Fig. 16. 



In practice, it is advantageous to limit the conversion bandwidth so 



