524 BELL SYSTEM TECHNICAL JOURNAL 



This expression for the gain-band figure of merit of a two-circuit, line- 

 to-line amplilier is particularly useful for grounded-cathode pentodes and 

 klystrons. It is the product of three factors. The first may be called the 

 electronic figure of merit because it depends only upon electron stream 

 parameters (ratio of transadmittance to mean capacitance of the elec- 

 tronic transducer Te). The second is the degradation factor giving the 

 effect of adding passive circuit capacitance both inside and outside the 

 bulb to the active capacitance already present in the electronic transducer. 

 The third factor, called the matching factor, depends only on the matching 

 conditions and on the arbitrary definition of bandwidth. If the band is 

 taken 6 db down (3 db for eacy circuit) and the tube input and output 

 are matched, the third factor is unity. 



In amplifiers using triodes and tetrodes in grid-return circuits, the Q 

 of the input circuit is usually very much smaller than that of the output. 

 Here it is appropriate to use the single-circuit limiting concept, with 

 Qu << Qie . Here a figure of merit independent of bandwidth is obtained 

 from the product of power gain and bandwidth: 



Top Bs = 



This expression for the gain-band figure of merit of a one-circuit, line- 

 to-line amplifier is also the product of three factors. The first is again the 

 intrinsic electronic figure of merit of the active transducer alone; the 

 second is the degradation produced by the addition of passive circuit 

 capacitance to the output and circuit loss to the input; the third is a 

 band-definition matching factor which is unity when the band is taken 3 

 db down and the tube is matched. 



In the application of the figures of merit, the third factors are usually 

 omitted, since they depend only on the matching conditions and on the 

 particular definitions of bandwidth used. 



Power-Band Figure of Merit 



In the problem of power output amplifier stages, the design specifica- 

 tion of greatest importance is the bandwidth over which a certain power 

 output can be obtained with a specified maximum of distortion. Of the 

 many methods of specifying distortion, one which is particularly useful 

 for microwave systems is known as the "compression". If the power gain 

 is plotted in decibels as a function of the power output, as shown in Fig. 



