194 BELL SYSTEM TECHNICAL JOURNAL 



The resistance of the coils was measured as described before l)y 

 taking resonance cur\es at different frequencies. The distributed 

 capacity of each coil was 14X10"'- farad (including tube capacii\). 

 By means of these values, the curve C was calculated.- The unknown 

 capacity coupling makes it impossible to predict the exact shape of a 

 transformer coupled amplifier from the constants of the circuits. 

 However, the calculated curve C (calculated for inducti\'e coupling 

 only) will give a general idea of the shape of an experimental cur\e A . 



Curves A and B, Fig. 10b, show the amplification curves for the 

 case of capacity coupling alone. A is the experimental and B the 

 calculated curve and they are seen to give fair agreement. The 

 coupling capacity was 21X10"'- farad and tiie distributed capacity 

 of the coils was 19.3X10"'^ farad, the increase, as compared with the 

 case of inductive coupling, being due to the ground capacities of the 

 coupling condenser. 



In connection with this type of amplifier it ma\' be mentioned that 

 a higher amplification naturally can be obtained if the plate of the 

 amplifier tube is connected across a part of the primary circuit onK , 

 maximum amplification corresponding to the circuit impedance 

 being equal to the plate impedance. However, the same effect will 

 take place here as was shown for the tuned circuit amplifier, namely, 

 that the band width will decrease with increase in amplification. 

 Using transformers at their natural frequency instead of coupled 

 tuned circuits with outside condensers will give broader bands or 

 higher amplifications corresponding to the single tuned circuit 

 amplifier. 



'The following two fortmilns hnvc been used for (.uliiilaling tlu' amplifuMtion of 

 the circuit shown in Fig. 9. 



Inductive Coupling,: 



1.1/ 1 1 



Amplification = 



e, Z uCIi^{l-u'L>C)+R'+j{uL'+Ii,R<o,C) 



— \''<"+(-i)' '"=«(■ + ("#))."'-'="'-("#)■(-'- -t)- 



Capucilive CoupHna: 



. ..^ . R + jwL 



.Amplification =n . , ■ ., 



.here .,=/.„(, -.V7.C+2^-o,^Z.cf, - ^^,^,) -,R{^,-[.). 



B=RRM-{i+'^) - ,,,(S..'L^) +"^('4)-;;;^" 



