MODULATION IN VACUUM TUBES 



453 



purposes, but for greater precision it may be desirable to use analytical 

 methods for the determination of the b coefficients. 



U. U. L- 



\ 



<i A 

 \«\ 



'\ ^S.^! 



\ '\ '^^" 

 \ \ ^ ( I 



<!\ 'N^_ ^^_ 



jSv )( >v "^ 



4,000 6,000 8,000 iqOOO 



LOAD RE5I5TANCE IN OHMS 



Fig. 4- 



IZ,000 H.OOO K>,000 



-Modulation coefficients for EL tube No. 109,150. Ec = - 9, Ep = 120 



Consideration of the expression for the second order coefficient, C2, 

 shows that the three terms of the numerator are all important, in 

 general, except that the last term is negligible at very low resistances. 



C2 = 



dEc. 



C,{R - Ro 



dfj. 



— RqCi- 



dRo 



dE,, 





(7) 



Now in amplifiers, the condition for maximum power delivered to 

 the load resistance at maximum gain demands equality of internal 

 and external resistances, and this coincides with the requirement for 

 minimum reflection coefficient "^ when the amplifier is connected to a 

 line of definite characteristic impedance. 



Under normal conditions, then, we have for the second order 

 coefficient 



Co^ 



3yU 



'dE,. 



fjr dRp 



1 



4i?o 



(8) 



in which the variation of /x with respect to Ep does not enter, the only 

 determining quantities being the variation of /x with respect to JSf„, 

 and the variation of Ro with respect to £;,„. The second order modula- 



^ The reflection coefficient is expressed as the quotient of the difference by the 

 sum of the two connected impedances. 



