56 BELL SYSTEM TECHNICAL JOURNAL 



The effect of distributed capacity in the windings (present especially 

 in the secondary because of its greater number of turns) is, more or 

 less, to shunt the high frequencies. This may be counteracted either 

 by the inductance in the primary winding or, if this is not sufficient, 

 by insertion of the proper inductance in series with the primary. It 

 may be said, in a general way, that the lower the ratio of a trans- 

 former the better suited its frequency characteristic will be to a wide 

 band of frequencies such as occurs in speech, and transformers with a 

 ratio of 1 to 4 are made which require no correcting provided they are 

 properly chosen with respect to the impedance characteristics of 

 associated tubes. 



The selective amplification of an amplifier for particular frequencies 

 may be increased by tuning one or more of the secondaries of the 

 interstage transformers with condensers. (See Equation 8a.) 



Due to the fact that there is an appreciable distributed capacity 

 between the primary and the secondary windings, an interstage 

 transformer supplies capacity coupling as well as inductive coupling 

 between adjacent stages, the phase of the capacity coupling being 

 independent of the direction of winding while the inductive coupling 

 is not. Therefore, the transformer may be so placed in the circuit 

 that these two effects either aid or oppose one another. In order to 

 secure the greatest amplification they should aid. 



The transformer used in speech frequency work is, in general, made 

 with an iron core; therefore, care should be taken to prevent the d.c. 

 component of I p magnetizing the core and reducing its efficiency. 

 One method of accomplishing this is shown in Fig. 26 in which the 

 d.c. component of I p is bypassed by a choke coil. In circuits in which 

 two or more interstage transformers are used, attention should be paid 

 to the danger of magnetic feed-back. This can be largely eliminated 

 by using transformers with closed magnetic circuits. Both the toroid 

 type core and the shell type core (commonly employed in power 

 transformers) have been found satisfactory, and especially the latter. 



22. Amplification of Higher Frequencies. The transformer coupled 

 amplifier is the type perhaps best suited to use at frequencies higher 

 than those of speech. As a special case the amplifier circuit of Fig. 27 

 will be considered first. This circuit contains a tuned output and 

 should be used only in case a single frequency or very narrow band of 

 frequencies is to be amplified but in this case will be found very 

 satisfactory. The inductance L may consist of two parallel windings, 

 insulated from each other to avoid any conductive connection be- 

 tween the plate battery and the output. Such an arrangement 

 would be desirable if the output went to a detecting tube or another 



