1066 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1954 



impedance, because it will not oscillate when its two terminals are 

 open circuited. 



THE E3 REPEATER UNIT 



The E3 repeater is essentially a two-terminal network the impedance 

 of which has a magnitude | Z \ and a positive phase angle that can vary 

 with increasing frequency from plus 90 degrees, or less, through plus 

 180 degrees to at least plus 270 degrees. This type of negative impedance 

 is shown in Fig. 6(b). It has been known as the shunt type because it 

 could be produced by connecting the output of an amplifier back in 

 shunt with its input. In more recent years it has become kno^\ai as the 

 short circuit stable type because it will not oscillate when its two termi- 

 nals are short circuited. 



THE NEGATIVE IMPEDANCE CONVERTER 



The amplifier circuits of both of these repeater units perform the 

 same function: that of a negative impedance converter. The operation 

 of such converters is illustrated in Fig. 7. 



Fig. 7(a) shows the converter as a four-terminal network having a 

 ratio of transformation k and a shift of phase through a negative angle 

 of approximately 180 degrees over the operating band of frequencies. 

 If, as shown, an impedance Zn is connected to terminals 3 and 4 then the 

 impedance seen at terminals 1 and 2 will be the impedance Z^ multi- 

 plied by the ratio h and shifted in phase through a negative angle of 180 

 degrees. This impedance will (over the frequency range of zero to in- 

 finity) fulfill the definition given for the impedance presented by the E2 

 repeater. Hence, Fig. 7(a) can represent the operation of the E2 repeater. 



IklAio^ZN 



c 

 |k| /i80°: 1 



(a) 



Fig. 7 — The Negative Impedance Converter: (a) E2, open circuit stable and 

 (b) E3, short circuit stable. 



