IMPROVEMENTS IN COMMUNICATION TRANSFORMERS 139 



there is shown the effect of using silicon steel as core material, which 

 was the practice in older transformers. 



An important limitation of high permeability alloys is their sensi- 

 tivity to mechanical strain which may seriously impair their magnetic 

 characteristics. Considerable care must be exercised to avoid strain 

 during assembly operations after laminations are annealed. Tele- 

 phone transformers are designed specially to provide a firm assembly 

 without mechanical strain, thereby retaining the high permeabilities 

 available. 



Increase in Voltage Amplification 

 As may be seen from the foregoing curves, the voltage amplification 

 of input transformers at the low end of the frequency band is directly 

 dependent on the permeability of the magnetic core material. At the 

 highest frequencies the voltage amplification of the above input trans- 

 formers is controlled by leakage and capacitances, the latter including 

 grid circuit capacitances as well as the transformer distributed capaci- 

 tances. Over a wide range in the central part of the frequency band 

 these effects are negligible and the transformer performs much as an 

 ideal transformer of the same ratio. By proper proportioning of the 

 leakage and capacitance effects, the shape of the characteristic may 

 be controlled to a certain measure at will. For example, a rising 

 voltage amplification-frequency characteristic can be obtained if de- 

 sired to correct for a falling characteristic of other parts of the amplifier. 



,-.65 



O fy fin 

 -0, 60 



5 2 

 |vj^50 





o 

 > z 



- 35 



100 



5000 10,000 20,000 



500 1000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 4 — Voltage amplification-frequency characteristic of an input transformer having 

 an impedance ratio of 10,000 to 1. 



In certain types of circuits the voltage amplification of input trans- 

 formers is at a high premium, such as in the amplification of low-energy 

 signals when a.-c. power is used for the tubes. Under these conditions 

 the tubes tend to introduce appreciable noise. A high-voltage ampli- 

 fication in the input transformer serves to raise the signal voltage at 

 the grid terminals so as to override the tube noises. Figure 4 shows 



