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BELL SYSTEM TECHNICAL JOURNAL 



is customary to make the retard and choke coils of the toroid or closed 

 core type. 



21. Transformer Coupled Amplifiers. From a theoretical point 

 of view the transformer coupled amplifier (Fig. 26) should be the 

 ideal type. By the proper choice of transformers it should be possi- 

 ble to match stages with respect to one another in such a way as to 

 obtain the greatest efficiency from tubes and batteries. The chief 

 advantage of transformer coupling lies in the fact that the input 

 voltage to the second stage may be made greater than the voltage 



Z Zi 



Fig. 26 



output of the first, and so on, at the same time that each tube operates 

 into a properly matched impedance to give maximum power output. 

 When uniform amplification over a relatively wide band of frequencies 

 is not required, the interstage transformer may be designed to step 

 up the voltage as many as 30 to 40 times. Other advantages are 

 the economical use of plate batteries (Sec. 14) and the elimination of 

 grid condenser and grid leak or high voltage grid battery. 



However, the difficulties attendant upon the design and making of 

 transformers are such that to realize the apparent advantages of this 

 type of circuit will require very careful planning. This will be illus- 



Fig. 26a 



trated by the following example of a transformer to handle the fre- 

 quencies of speech. Given an interstage transformer as in Fig. 26a, 

 we will assume that the transformer works out of a tube impedance 

 r p and into a grid circuit impedance which has infinite resistance. 



