NEGATIVE IMPEDANCE TELEPHONE REPEATERS 



100.-) 



Theory 



negative imi'kdaxc'e concept 



Both the E2 and E8 repeater elements contain an aniplilier ha\ing 

 multiple feedback paths. The operation of an amplifier circuit of this 

 type can be explained by classical feedback theory. However, experience 

 with the El repeater over the past four years has shown the value of 

 using a negative impedance concept in engineering such a device. Hence, 

 in the explanation of operation given here, the repeater units will be 

 treated simply as two-terminal networks which have negative impedance 

 inputs over the freciuency band of interest. The effect of introducing 

 these impedances into telephone circuits can then be computed by the 

 same simple network theory used to determine the effects of passive 

 impedances. 



THE E2 REPEATER UNIT 



The E2 repeater is essentially a two-terminal network the impedance 

 of which has a magnitude | Z \ and a negati\-e phase angle that can vary 

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

 180 degrees to at least minus 270 degrees. This type of negative im- 

 pedance is shown in the diagram of Fig. 6(a). It has been known for 

 many years as the series type because it could be produced by con- 

 necting the output of an amplifier back in series with its input. More 

 recently it has come to be known as an open circuit stable, negative 



-R 



R -R 



Fig. 6 — The two t\j)cs of negative impedance: (a) i)\w\\ Circuit Stahie and 

 (!)) Short Circuit Stable. 



