392 BELL SYSTEM TECHNICAL JOURNAL ■ 



Neglecting the stray capacitances between the lead wires and also the edge 

 efifects, the greater proportion of the cold capacitance (input plus output) 

 in Farads can be approximated by 



Co = .0885/1 (i + 1 + 1^ X 10-^' . . .(10) 



\a c) 



where "a" and "6" have the same meaning as in (7) and "c" is the spacing 

 between the plate and the suppressor (or screen in the case of a tetrode). 

 This is of course a highly idealized case. The cathode, the grids, and the 

 plate, are each assumed to be plane conductors of infinitesimal thickness, 

 each having an area equal to the active area of the structure.* The band 

 merit then becomes, 



^ G„ 4.74M7J'' X 10' ,,,. 



^" = 2^0= —^^ ^ ^^ ^—"^^ ^^^^ 



\a b c/\ JU12 a / 



With a given cathode current, the factor M increases as the screen current is 

 reduced. The use of small wires for the screen grid is an important factor 

 in obtaining minimum screen current. 



/o , the useful cathode current density, is limited by the emission capabili- 

 ties of the cathode. In practice it is necessary to operate in a region con- 

 siderably below the maximum available emission to avoid excessive changes 

 in transconductance which would result from variations in cathode activity 

 with time. Also, the shot noise will begin to rise when the region of tempera- 

 ture-limited operation is approached. It should be noted that Bo is inde- 

 pendent of the area A . 



Taking reasonable values for M, /o and jui2 , {M = 0.75, h = 50 ma/cfn^, 

 Mi2 = 25), equation (11) becomes 



&= ^ '-^'.X.'"' (12) 



^*"(^:- + !)(' + ■- "4-0 



The curves in Figs. 6, 7 and 8 show how Bq varies with each of the variables 

 "a", "6" and "c" when a constant value is assigned to the other two. The 

 band merit shown on these curves is considerably greater than that which 

 can be realized in an actual circuit of the simple band-pass type assumed 

 because the stray capacitances in the tube, the socket, the circuit elements, 

 and the wiring increase the total interstage capacitance substantially. Also, 

 the grid-cathode capacitance is substantially higher under normal operating 



* This assumption is obviougly not true, particularly in the case of the suppressor grid, 

 but, by simply regarding the effective suppressor-plate spacing as somewhat greater than 

 the actual spacing, the assumption becomes useful. 



