The Operation of Vacuum Tubes as Class B and 

 Class C Amplifiers* 



By C. E. FAY 



A simple theoretical development of the action of a vacuum tube and its 

 associated circuit when used as a Class B or Class C amplifier is given. An 

 expression for the power output is obtained and the conditions for maximum 

 outputs are indicated. The way in which the tuned plate circuit filters out 

 the harmonics in the pulsating plate current wave is illustrated by an hy- 

 pothetical example. A set of dynamic output current characteristics is 

 developed graphically from a set of static characteristics. The Class B 

 dynamic curves are found to give a better approximation to a straight line 

 than the Class C curves because of a reversed curvature which appears at 

 the lower ends. It is pointed out that the screen grid tube should function 

 similarly to a high m three-element tube in this type of operation. Experi- 

 mental dynamic characteristics of a three-element tube, Western Electric 

 251-A, and of a screen grid tube, Western Electric 278-A, of identical di- 

 mensions are shown which verify the theoretical results. The screen grid 

 tube gives about the same output and efficiency as the three-element tube, 

 but its dynamic characteristic tends to bend more rapidly at the upper end. 



Introduction 



THE majority of modern radio telephone installations in this country 

 are designed for modulation at a low power level and amplification 

 of the modulated carrier, so as to obtain up to 100 per cent modulation 

 in the output stage. As far as the writer is aware there seems to be 

 no very comprehensive material dealing with this phase of vacuum 

 tube operation available from past publications. A treatment of the 

 operation of such amplifiers seems particularly desirable from the 

 standpoint of the design of vacuum tubes for such service. 



Some of the fundamental considerations regarding Class B or C 

 operation were given by Morecroft and Friis.^ and later, a more com- 

 plete analysis of power oscillators by Prince.- Both of these, however, 

 were primarily concerned with the attainment of steady output at high 

 efficiency. Other papers 3. 4, 5 Qf j^ore recent date have touched some- 

 what upon the subject. 



* To appear in Proc. I. R. E., March, 1932. 



1 J. H. Morecroft and H. T. Friis, "The Vacuum Tube as a Generator of Alter- 

 nating Current Power," Transactions A. I. E. E., Vol. 38, No. 2, Oct., 1919. 



2 D. C. Prince, "Vacuum Tubes as Power Oscillators," Proc. I. R. E., Vol. 11, 

 Nos. 3, 4, 5, June, Aug., Oct., 1923. . t^ j. 



3 A. A. Oswald and J. C. Schelleng, "Power Amplifiers in Transatlantic Radio 

 Telephony," Proc. /. i?. £., Vol. 13, No. 3, June, 1925. „ ^ ,,, ,, ^t ^ 



" E. E. Spitzer, "Grid Losses in Power Amplifiers," Proc. I. R. E., Vol. 17, No. 6, 



June, 1929. . . . . ^ . r ^ • j - n 



6 Y Kusunose, "Calculation of Characteristics and the Design of Tnodes, Proc. 

 I. R. E., Vol. 17, No. 10, October, 1929. 



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