CATHODE FOLLOWER 



argument, if we are working where the input resistance is negative, then it 

 is 20 times higher in the cathode follower configuration than as an amphfier, 

 and the amount of external positive resistance required to ensure stability 

 can be 20 times higher. This much larger external resistance can often be 

 the source resistance itself, and there is then no need to provide a special 

 resistor (/?' in Figure 11.11) to ensure stabiHty. Thus, whatever the bias 

 used, it is true to say that the cathode follower has a high input resistance. 



A.c. coupled cathode follower 



When cathode followers appear in a.c. coupled amplifiers, the arrangement 

 is slightly different {Figure 11.12). Automatic steady bias is developed 



HT+ 



Figure 11.12 



across /?[.Q.. Bg, because it is returned to a point which moves with the 

 cathode and not to earth, is the ordinary upper limit of grid resistance 

 allowed by the valve makers. For a gain of 0-95 as before, however, the 

 effective value of Rg as seen from the source is 20 Rg, so the input resistance 

 is very high. Furthermore, 20 R,j is the resistance value relevant to the 

 calculation of required coupling CR product, so Q can be 1/20 of the 

 value it would have to have if the valve were an amphfier. 



Input capacitance 



With the cathode follower circuit the capacitance between grid and anode 

 remains unaffected at Cg^ (there is no Miller effect because the anode poten- 

 tial is fixed) but the effective capacitance between grid and cathode is much 

 reduced (Figure 11.13). If there is a change of grid potential dVg and the 

 cathode follower gain is 0-95, ^F^. = 0-95^F, and dVg,, = 0-05 SVg. The 

 displaced charge is 0-05 dVgCg^. so the apparent grid-cathode capacitance 

 is 



Displaced charge 0-05 dV„Cn 



g^gk 



dV„ 



= 0-05 C 



gk- 



Change of grid voltage 



The grid-cathode capacitance is therefore reduced 20 times. 



With a pentode cathode follower the gain without feedback is higher than 

 with a triode, so the gain with feedback is more nearly unity and the improve- 

 ment in input resistance and grid-cathode reactance even better. The gain 



171 



