510 BELL SYSTEM TECHNICAL JOURNAL 



Another difference lies in the fact that both ampHfiers work at the 

 same time. Even though the echo waves are cancelled out their 

 energy is not lost, but is added to the amplified wave. The output 

 current is twice, and the output power is four times what either 21 -type 

 circuit acting alone would send toward the listener. In the 21 or 22- 

 type circuit, only half the output power of one amplifier reaches the 

 line, the other half being absorbed in the opposite line or in the net- 

 work. The output power is, therefore, 3 db less than that which the 

 amplifier actually produces. In the twin 21-type circuit one-half the 

 output power of each amplifier is also absorbed in a network, but the 

 remaining halves are combined in the output wave. Consequently, 

 the total output is equal to that of one amplifier. For this reason, 

 with a given size of vacuum tube, the twin 21-type circuit can deliver 

 twice as much useful power, or 3 db larger volume to the line, than 

 either the 21 or 22-type repeater. 



PusH-PuLL Effect 



If the connection between the line hybrid coil and either of the 21- 

 type circuits is transposed, the directions of current flow in the 21-type 

 circuit are all reversed, but the directions of the input and output 

 currents in the line conductors are not affected. If the amplifiers are 

 perfect, such a transposition will have no effect upon the operation of 

 the twin 21-type circuit. When vacuum tubes are used as amplifiers, 

 however, there is a certain amount of distortion due to the curvature 

 of the operating characteristics of the tubes. 



If the connections are so arranged that the grids of the tubes in both 

 of the 21-type circuits receive positive potentials from the incoming 

 wave during the same half-cycle, this distortion will appear in the out- 

 put wave of the twin 21-type circuit. If, for example, the input wave 

 is a pure sinusoid, the output wave will contain a series of harmonics. 

 Some of these harmonics will be of even number, principally the second 

 harmonic, and correspond to a difference of the shapes of the positive 

 and negative half-cycles. 



Transposing the connection of one of the 21-type circuits as described 

 above causes one of the grids to receive positive potential from the 

 input wave at the same time that the other grid receives negative 

 potential. This reverses the phase of the even numbered harmonics 

 from one of the 21-type circuits with respect to those from the other, 

 and so eliminates the even numbered harmonics from the output wave 

 of the twin 21-type circuit. This result is similar to that obtained by 

 means of the familiar push-pull arrangement of vacuum tubes used 

 in an amplifier to reduce distortion, but no increase of the number of 

 tubes is required. 



