NEGATIVE VOLTAGE FEEDBACK AND THE STABILIZED GAIN AMPLIFIER 



without feedback can easily be 100, so that cathode follower gain will be 

 100/(1 + 100) == 0-99. The factor by which the input resistance and grid- 

 cathode reactance is improved is now approximately 100. Further, since 

 the pentode has a very low Cg„, there is also a big increase in the grid-anode 

 reactance. 



HT+ 



SV. 



I 



\ 



\cx ^Vf, 



Cgk 



HT- 

 Figure 11.13 



There is a minor complication. Pentode action depends on the potential 

 difference between cathode and screen remaining constant. Cathode-follower 

 action requires that the load current be equal to the anode current, or at 

 least different from it only by a constant amount {Figure 11.14). This means 



HT+ 



Anode 

 11 current 



HT+ 



Screen 

 current 



Anode+screen 

 current 



HT+ 



Anode 

 current 



Anode 

 current 



HT- 



Figure 11.14 



Figure 11.15 



Figure 11.16 



that the screen current must be held constant, which again requires that the 

 screen-cathode voltage be somehow held steady. In the a.c. coupled pentode 

 cathode follower this may be achieved by returning the screen decoupling 

 capacitance to the cathode and not to earth {Figure 11.15). In the direct 

 coupled pentode cathode follower the screen is most conveniently supplied 



172 



