526 BELL SYSTEM TECHNICAL JOURNAL 



Tlie power output depends on operating parameters thus: 



As the output power level is continually raised, more and more cur- 

 rent is required to drive the load, until finally the non-linear distortion 

 limit is reached. The maximum output current is therefore limited to a 

 certain proportion of the direct current hn , thus: 



hm = hjrFiO F,(d.,) (Al-2n) 



where F{C) shows the dependence upon the compression C and will 

 naturally be the larger, the more the allowable compression. ^2(^2) indi- 

 cates a dependence upon output transit angle; it is the output gap cou- 

 pling coefficient. 



The power output depends also upon the output circuit conductance 

 G2 and can be greater if G2 is smaller. However, a smaller Go implies a 

 smaller bandwidth. It results that the power is inversely proportional to 

 the bandwidth of the output circuit, or in other words, the product of 

 power output by the bandwidth of the output circuit is a constant— a 

 figure of merit of the tube. As in the case of the gain-band merit, this 

 also can be broken up into factors: 



Po • Bn 



( i\.F'{c)Fl{d,) \ ( 1 \ ( iVm - l \ , . 



V 47rC22 / \1 + C,,/Cj V 1 + M2 / ^ ^ 



This expression for the power-band figure of merit is the product of 

 three factors. The first is the intrinsic figure of merit of the active trans- 

 ducer alone; the second is the degradation caused by the addition of pas- 

 sive circuit capacitance to the output circuit; the third is a band defini- 

 tion — matching factor which is unity when the output is matched and the 

 band of the output circuit is taken 3 db down. 



The power-band computation does not depend upon the input circuit. 

 Variations in the latter affect the gain of the amplifier, but not its over- 

 load point. Accordingly in the power band formula only properties of the 

 tube and its output circuit appear. When feedback has to be considered, 

 then the input circuit also affects the power, and the analysis becomes more 

 complicated. 



We have now three figures of merit: namely, two gain-band products 

 applying to different kinds of amphfiers, and one power-band product. 

 They relate the performance of an amplifier to certain internal parameters. 

 For wide band service, the tube design should make the appropriate 

 figure of merit as large as practicable, 



