THERMIONIC VACUUM TUBES 



57 



amplifying tube. By using a variable condenser C, the frequency of 

 maximum amplification can be readily shifted but for any one setting 

 the amplification will be as shown by curve A of Fig. 28. For maxi- 



C -r Output 



Fig. 27 



mum output, the position of the tap A should be set so that the 

 impedance of the tuned circuit LC as seen from the tube is equal to 

 the output impedance of the tube. Tuned circuit amplifiers are 

 especially adapted for amplification at very high frequencies (above 

 2,000,000 cycles), where the effect of the capacities between the 

 elements of the tubes makes other types of amplifiers very inefficient. 

 The amplification curve A will be broadened when more and more 

 turns are added to the inductance L and the capacity of the tuning 

 condenser C is diminished due to the effect of the distributed capacity 



frequency — 

 Fig. 28 



of the coil. Curve B gives the amplification for a circuit where the 

 condenser C is omitted in which case it becomes a retard coupled 

 amplifier. Maximum amplification occurs at the natural frequency 

 of the coil including the capacity effects of the leads and elements of 

 the tubes. 



In case a relatively wide band of frequencies is to be amplified, 

 transformer coupling is usually resorted to, and given suitably de- 

 signed transformers one to two octaves can be amplified with very 

 fair uniformity at frequencies between 100,000 to 2,000,000 and 

 four to five octaves at frequencies below 100,000 cycles. Use of 

 transformer coupling will broaden the characteristic of the amplifier 

 as shown in curve C, Fig. 28, the exact shape of this curve being 

 largely dependent upon the design of the transformers employed. 



