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BELL SYSTEM TECHNICAL JOURNAL 



formers, since there the core characteristics limit the transmission 

 only at low frequencies where the effective permeability is high. 



At higher frequencies the voltage amplification is severely limited 

 also by the grid circuit and transformer capacitances. It has been 

 found advantageous to add correcting elements, such as inductances 

 and capacitances, to increase the gain ordinarily available. These 

 added elements and the equivalent elements in the transformer are 

 designed together as configurations similar to low-pass filter sections 

 terminated midshunt in the grid circuit capacitance. Figure 8 illus- 



o 



P 

 Oca 



500 1000 10,000 100,000 



FREQUENCY IN CYCLES PER SECOND 



1,000,000 



Fig. 8 — Transmission loss-frequency characteristic of a transformer designed to 

 transmit high frequencies in addition to audio frequencies. 



trates the performance of a transformer designed for certain high fre- 

 quency transmission experiments. The performance of this trans- 

 former in the frequency range of 30-500,000 cycles per second is similar 

 to that of earlier types in the range of 30-60,000 cycles per second. 



In most telephone applications the design of input transformers is 

 complicated greatly by circuit functional requirements in addition to 

 those of direct transmission. For example, as discussed in a succeed- 

 ing section of this paper, phase distortion requirements demand self- 

 impedances far higher than those required by transmission loss con- 

 siderations, thermal ^ noise requirements demand lower dissipations, 

 and impedance requirements limit the voltage amplification obtain- 

 able. For feed-back type amplifiers ^ the input transformers in addi- 

 tion to the forward amplification must meet transmission and phase 

 requirements in the regenerative path. These special requirements 

 apply not only for the transmitted frequency band but also at fre- 

 quencies remote from this band in order to insure stability of the 

 amplifier. 



Reduction in Phase Distortion 



Since transformers are reactive devices, they introduce phase shift 

 in the circuits in which they are used. If the phase shift introduced 

 be a linear function of the frequency it will not produce any distortion 



I 



