100 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



verter a resistance of 1800 ohms (not shown in Fig. 6). The final result is 

 that the impedance seen, looking into terminals 1 and 2 of the El converter 

 when 1800 ohms plus a network Zn is connected to terminals 3 and 4, 

 equals — O.lZjv within a reasonable percentage of error over the frequency 

 range from about 300 to 3500 c.p.s. for values of negative impedance from 

 about 100 to 2000 ohms. 



In the practical design two line windings instead of the one shown con- 

 nected between terminals 1 and 2 in Fig. 6(a) are provided on transformer T. 

 In practice one of these windings is inserted in each side of the telephone 

 line in a balanced arrangement. Terminals 1 and 2 are thus effectively con- 

 nected in series with the line, and the El repeater presents to the telephone 

 line a reversed voltage type of negative impedance (— F/I), which is the 

 means of introducing additional power in the line thereby providing a 

 transmission gain. The value of the negative impedance is controlled by a 



NETWORK 



EH 



AMPLIFIER 



ImJ 



■^w^ 



LINE 1 TRANSFORMER LINE 2 



rw^ 



Fig. 7 — El telephone repeater. 



network connected to terminals 3 and 4. Thus the El repeater consists of a 

 transformer, an amplifier unit and a gain adjusting network (Fig. 7). The 

 transformer, the amplifier unit and part of the network make up the nega- 

 tive impedance converter under discussion. 



For overload conditions the action of a repeater of this kind differs 

 markedly from what might be expected from a conventional amplifier. 

 What can be expected of a conventional amplifier is well known. An idea 

 as to the performance of a negative impedance converter under overload 

 conditions can be had from the following example: Assume that terminals 

 1 and 2 of the converter are connected in series with a telephone line and a 

 network is connected to terminals 3 and 4 so that a reversed voltage type 

 of negative impedance of a value less than the line impedance is inserted in 

 the line. The combination described will be stable, and some transmission 

 gain will be provided by this negative impedance. If now the volume of 

 speech on the line is increased beyond the overload point the result will be 

 a noticeable reduction in the amount of negative impedance, and a conse- 



