INTERSTAGE COUPLINGS 



capacitance is low due to the substantial suppression of Miller effect. (2) is a 

 much greater effect than (1), and as F^ has itself to be coupled to a source 

 possessing internal resistance ampHfiers often employ pentodes throughout. 

 With direct coupled amphfiers the equivalent circuits at high frequencies 

 are similar, the climbing and neon-coupled types performing best. With 

 couphng batteries the upper cut-off frequency is liable to be somewhat low 

 because of the stray capacitance of the bulky battery to earth. The potential 

 divider type requires rather more comment. Referring back to Figure 9.3 but 

 assuming Fj and V^ to be pentodes, the equivalent circuit at high frequencies 

 is Figure 9.15. Sectioning the circuit along the dashed line and applying 



ffm<SK 



Vc grid 



n->-A)Cga 



Figure 9.15 



Rl r, 



fl^/-'''*^- 



——1^2 9'"'^ 



^2 -L 



Cs*Cg^*(uA)C, 



■go 



ir^ 



Figure 9.16 

 HT+ HT* 



^ n^C, <R 



S^2 



HT- 

 Figure 9.17 



Thevenin's theorem to the part to the left, we get Figure 9.16. Clearly since — 

 as we have seen — R^ and R^, have to be made much larger than Rj^ the source 

 resistance is higher than in the other systems and the upper cut-off frequency 

 will be correspondingly low. 



The solution is to connect a compensating capacitance Cq across R^ as in 

 Figure 9.17. Lumping V^ input capacitances together as Cm, the equivalent 



156 



