NEGATIVE IMPEDANCES AND THE TWIN 21 -TYPE REPEATER 509 



When two condensers are used in series, as shown, to keep the circuit 

 balanced, each one must, of course, have a capacitance 2Ci. 



It would be possible to carry this principle still further, if necessary, 

 so that anything introduced between the series 21-type circuit and the 

 line which results in adding impedance in series or shunt with the 

 series 21-type circuit can be matched by adding suitable impedance in 

 shunt or series, respectively, with the bridged 21-type circuit. Simi- 

 larly, anything which affects the impedance of the bridged circuit can 

 be matched by a corresponding addition to the series circuit. In 

 order for these additional impedances to match, the following relation 

 must be established at all frequencies in the useful range: 



zs X zs= R\ (33) 



in which zs is an impedance effectively in series, or parallel, with the 

 series 21-type circuit and Zb an impedance effectively in parallel, or 

 series, respectively, with the bridged 21-type circuit. The value Zs 

 is referred to the line windings of the line hybrid coil, that is, if the 

 element contributing this impedance is connected to the drop winding 

 of the line hybrid coil its actual impedance must be multiplied by the 

 square of the turn ratio of the entire line winding to the drop winding 

 to obtain Zs. 



If more than one part of the series 21-type circuit must be compen- 

 sated by corresponding parts of the shunt circuit, it is necessary that 

 the corresponding parts be arranged in the same order between the 

 line and the 21-type circuits. 



Special Properties of the Twin 21-Type Circuit 



The twin 21-type repeater differs in a number of important respects 

 from the 22-type repeater and others that have been used or proposed 

 in the past. It is essentially a network of impedances two of which 

 include negative resistance components. These are the two 21-type 

 circuits. Each 21-type circuit is connected to the line by only one 

 pair of terminals through which the input w^ave enters and the amplified 

 wave leaves it; hence, it may be treated as a single impedance which 

 has a negative resistance component. It follows from this that the 

 twin 21-type circuit follows the reciprocal laiv, and that the gain 

 at any frequency is the same for both directions of transmission. This 

 is true even if the two 21-type circuits are not set for the same gain. 

 If the gains of the two circuits are different the amplified current wave 

 will be the sum of the current waves from the two 21-type circuits (as 

 measured in milliamperes or other current units) and an echo equal to 

 the difference will travel toward the speaker. 



