FREQUENCY RESPONSE OF INTERVALVE COUPLINGS 



oj = l[( CRg) and is asymptotic to a slope of 6 dB octave. At high frequencie s 

 the response of both types of coupUng falls off, also at 6 dB octave, for a 

 simple low-pass filter is made by Kg grid- and stray-capacitance in conjunction 

 with the effective internal resistance of the source driving it. Consider the a.c. 



'"a 





c 



-j-^V^gnd 



f^^gQ 



U. i- 



*-tr^ 



Figure 9.11 



coupled case. The equivalent circuit for a triode V^ is Figure 9.11, as we have 

 seen. At high frequencies the reactance of C is negligible but the reactance 

 of the shunt capacitance is not. This shunt capacitance is made up of Fo input 

 capacitance, C^^, + (1 + A)Cga, plus Q, the stray wiring capacitances to 

 earth. Rg is large compared with i?^ and may be neglected (since it is now 

 virtually in parallel with it) so the equivalent circuit reduces to Figure 9.12. 





-\AA- 



Rl-^'o 





X 



,Cs + Cgl< 



'■^n*A)Cga 



' 1^2 grid 



Figure 9.12 



Figure 9.13 



If Ki is a pentode we have Figure 9.13 which reduces to Figure 9.14. It is 

 clear that Figures 9.12 and 9.14 are simple low-pass filters whose performance 

 may be calculated in the usual way. 

 Evidently the high frequency response is best when: (1) r„ and the load 



Rl 



^yggmRC' 



■1^2 grid 



Ci*Cgk 



Figure 9.14 



resistance of V-^ are low; (2) the input capacitance of V^ and the stray 

 capacitances are low. (1) is achieved by making V-^ a triode rather than a 

 pentode. (2) is achieved by making V^ a pentode, because pentode input 



155 



