NEGATIVE IMPEDANCE TELEPHONE REPEATERS 1070 

 THE E3 TELEPHONE REPEATER 



The E3 repeater is a two-terminal high impedance device designed to l)e 

 bridged across the hne in contrast with the E2 repeater which is a low 

 impedance series connected device. Scliematically shown in Fig. 17, 

 its circnit is considerably different from that of the E2. 'I'he l)ridged 

 telephone^ line and an adjustable network form two arms of a bridge 

 connected to output and input circuits of a two-stage amplifier. 



The bridge is so connected that a fairly large proportion of the am- 

 plifier output current flows into the telephone line and a fairly large 

 proportion of the original line voltage is developed between the grid 

 and cathode of the first tube since these circuits are connected to non- 

 conjugate points of the bridge. The output and input terminals of the 

 amplifier are connected to conjugate points of the bridge in such a 

 manner than when the bridge circuit is balanced no feedback is effective 

 at the input. 



It has been shown, in the preceding Section, that the negative im- 

 pedance generated by this form of converter is equal to the network 

 impedance Zy divided by a conversion factor. To obtain a practical 

 design of the E3 repeater for the faithful conversion of the network 

 impedances, with a minimum of spurious components, it is necessary 

 to balance out, as nearly as possible, all converted circuit elements 

 associated with the output bridge and connections to the line. Accord- 

 ingly, the two line capacitors are balanced out by a network capacitor. 

 The battery supply resistors, the resistor and capacitor of the plate- 

 battery filter, and the plate-load resistor of the first-amplifier stage are 

 all balanced out in the network by placing suitable values of resistors 

 and capacitors in the cathode circuit of the second-amplifier stage. All 

 of these elements combine to form an equivalent two-teiTninal network. 



An ideal negative impedance device would convert any impedance 

 in the network over a wide frequency band but it is advantageous to 

 limit the negative impedance, in so far as practicable, to the frequency 

 bandwidth required by the particular application. This is accomplished 

 primarily in the network associated with the converter. The conversion 

 bandwidth of the E3 repeater is restricted at the low frequencies by the 

 design of the resistive-capacitive feedback coupling network, between 

 the output bridge and the input grid, and at the high frequencies by 

 the shunt capacitance connected across one resistive arm of the bridge 

 circuit. The circuit is shown in Fig. 18. 



The final negative impedance shunted across the line is equal to 

 — Zjv/0.94, within ±2.5 per cent, over the frequency range of 200 to 

 5,000 cycles. The magnitude and phase of the negative impedances are 



