ULTRA-HIGH-FREQUENCY VACUUM TUBES 



659 



It was shown in (90) that CgjCp is the static, or " cut-off " ampli- 

 fication factor, Mo' of the tube. Again, y = Cc/Cp so that 



Vao = 



VpO + Mo VgO [ • J. \ \ 



-. — j rn (approximately). 



1 + Mo + >' ^ 



When the cathode is heated so that electrons flow, the potential Va 

 becomes depressed below this value which should be used only in 

 forming rough estimates. 



^GRID 



CATHODE 



Co 



Cp 



— o 

 PLATE 

 Vp 



Fig. 2. — Equivalent network of negative grid triode in the absence of electrons. 



A more accurate formula for Va involves the transit time ratio /i, 

 but may be obtained as follows : The plate current of an ideal negative 

 grid triode is proportional to (Fpo + M^oo)^^^ so that 



Similarly 



— ^ (^po + Mo^go) 



2 F.40 



3 h 



From these two equations together with (93) is obtained 



VpO -f MoFpo 



Vao = 



Mo + |(i+3')(i + /^) -\a + hy 



which may also be written 



Vao^ 



VpO + MoFgO 



1 + Mo + l^d + h) - \}H6 -f 4//. + h'-) 



(94) 



This is in a form for comparison with the appro.ximate equation 

 above and shows the modification produced by the presence of elec- 

 trons. 



