494 BELL SYSTEM TECHNICAL JOURNAL 



This 21 -type circuit is connected between two equal resistances, Rq. 

 Ri = 3^i?o and Ri = Ro, assuming that the hybrid coil is designed for 

 equal impedances at the two pairs of line terminals and the drop 

 terminals. If an e.m.f. E acts in series with the resistance Ro at the 

 left side of the repeater and the mutual impedance AI of the amplifier 

 is zero, a current, 



/. = 2i. (13) 



flows at the left hand terminals 5, 8 of the hybrid coil and in the input 

 circuit of the amplifier. One-half of the power entering the repeater 

 is absorbed in the input resistance Ri of the amplifier and the other 

 half is absorbed in the output impedance i?2- In accordance with a 

 well known property of the hybrid coil, no current will flow in the 

 right-hand resistance Ro. At a given instant this input current may 

 be assumed to have the direction indicated by the short arrows /i. 

 By increasing M the amplifier can be made active, causing an amplified 

 current /o to flow in series through the line windings of the hybrid coil 

 and the connected resistances Ro. By throwing the reversing switch to 

 one side I^ may be made to flow in the same direction as /i at the ter- 

 minals 5, 8 as indicated by the long arrows marked I2. For con- 

 venience, this will be referred to as the "direct connection." Changing 

 the reversing switch changes the direction of I2 with respect to /i, 

 giving the "reverse connection." As the hybrid coil is balanced, the 

 output power of the amplifier does not react upon the input circuit. 

 Putting A for the amplifying ratio of the 21 -type circuit, 



^ = ^; • (14) 



The total current flowing at the terminals 5, 8 is: 



/o= /l+/2 = ^(l+^), (15) 



ZKo 



and the active resistance of the 21 -type circuit is: 



As the amplification is increased, the current Iq increases while r falls 

 to zero and becomes negative, thus exhibiting series characteristics. 

 If A is increased without limit, r approaches —Rq in magnitude but 



