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BELL SYSTEM TECHNICAL JOURNAL 



For frequencies up to about 100,000 cycles, transformers with iron 

 cores of the ring type are suitable and are preferably enclosed in 

 metal covers which are grounded. A transformer suitable for fre- 

 quencies higher than 100,000 cycles may consist of two choke coils 

 (one to two inches in diameter) of very fine wire, these coils being 

 mounted close together on a suitable form. The natural frequency 

 of the coils will approximately determine the middle of the band of 

 frequencies which are amplified and the coupling between the two 

 coils will determine the width of the band, closer coupling resulting 

 in a wider band. The coupling is generally a combination of electro- 

 static and electromagnetic coupling and therefore, in connecting all 

 transformers for high frequency uses, it is essential to establish the 

 proper phase relations between them (see paragraph 21). Each 

 stage of the amplifier should be shielded as shown in Fig. 29 although 

 in certain cases it may be dispensed with. The shielding should 



Shielding 



Fig. 29 



consist of brass or copper sheeting made into boxes with well-soldered 

 joints and tightly fitting covers. Holes through the shielding should 

 be just large enough to pass the insulation of the wires. 



A common plate battery may safely be used for four or more stages 

 provided a condenser is placed across the terminals of the battery 

 as shown in Fig. 29. This condenser should have a capacity large 

 enough to offer practically no impedance to the high frequency cur- 

 rents, and its use may be desirable although the plate battery is 

 common to but two stages. FJse of a common grid battery, as shown, 

 introduces a small feed-back from the second stage to the first. This 

 feed-back may be either positive or negative, depending upon the 

 phase relations in the intermediate transformer and may be elimi- 

 nated by placing a condenser across the grid battery terminals. 



