DESIGN FACTORS OF THE 1553 TRIODE 499 



grounded-grid circuits, the situation is different because the Q of the 

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

 a figure of merit independent of bandwidth is obtained from the product 

 of power gain and bandwidth: 



iro|'5 = I I'2ir/47r6i,Cout (2) 



Here Gi„ is the total conductance of the input circuit, including tube con- 

 tributions*. The gain is again measured with the tube matched at an 

 arbitrary admittance level Gq. The band, being now limited by only one 

 tuned circuit, is somewhat different in shape from the above, and is taken 

 3 db down. 



While each figure of merit gives an unequivocal rating of tubes of 

 appropriate type, the intercomparison of the two types still depends on 

 the bandwidth. In particular, as the band is widened, the two-circuit type 

 (klystron) loses gain at the rate of 6 db per octave of bandwidth, while the 

 one-circuit type (triode) loses only 3 db per octave. Consequently, if the 

 two devices start with equal gains at some narrow bandwidth, the triode 

 rapidly pulls ahead in gain as the bandwidth is increased. 



The figure of merit equation (1) states that improved klystron per- 

 formance implies either an increase in transadmittance F21 or a decrease in 

 the band-limiting capacitances C\n or Cout • According to the simplest 

 klystron bunching concept,* the transconductance of such a tube may be 

 increased indefinitely simply by making the drift time longer. Unfor- 

 tunately, this simple kinetic picture does not take account of the mutually 

 repulsive space-charge effects which set an upper limit to the useful drift 

 time by debunching the electrons.^ For a 2000- volt beam in the 4000- 

 megacycle range, this limit is approximately three micromhos per milli- 

 ampere. The 402 A tube used in the New York to Boston system has 

 already approached this limit within a factor of two. Since the capaci- 

 tances are also quite small, the prospect is quite dubious for any con- 

 siderable improvement in gain-band merit if the simple klystron type of 

 operation were to be used. 



Improvements are possible in a klystron by changing the manner of 

 operation so as to lower the drift voltage Fo , because the aforesaid trans- 

 admittance limit is proportional to Fo~" .* This prospect is also relatively 

 unattractive. To get transadmittance values anywhere near the triode 

 would require low voltages and close spacings somewhat like the latter, 

 and would encounter space-charge difficulties involved in handling a large 

 current in a low-voltage drift space. Furthermore, the tube would be more 



< D. L. Webster, Jour. App. Phys. 10, 501-508 (July 1939). 



» S. Ramo, Proc. I. R. E. 27, 757-763 (December 1939). 



* The value of n may vary between J/4 and ^^. See reference 5, 



