CATHODE FOLLOWER 



CATHODE FOLLOWER 



When i?^ = the whole of the load resistance is between the cathode and 

 negative supply so that the feedback fraction is 1 and the gain is 1 approxi- 

 mately or Al(l + A) exactly (Figure 11.9). If we have the same valve as 

 before and R^ = 10,000 Q, 



Rj^ _ 10,000 _ 



"^^^ ^ ^a + Rk ~ 10.000 + 10,000 " 



and when [x has fallen to half its proper value A = 12-5. The gain alters 

 from 25/(1 + 25) to 12-5/(1 + 12-5) or from 0-945 to 0-93, a change of only 



HT + 



1-5 per cent. Undoubtedly a very high degree of gain stabilization; it may 

 well be asked, however, what is the point of taking all this trouble with a 

 device whose gain is less than 1, which in fact attenuates slightly. The 

 answer is that the cathode follower exhibits par excellence two valuable 

 results of applying negative feedback which we have not so far mentioned: 

 high input impedance and low output impedance. 



Input impedance 



This comprises two elements in parallel, the input capacitance and the 

 input resistance. 



Input resistance — We have to digress for a moment to properties of valves 

 in general. Up till now we have assumed that the input resistance of a valve 

 is infinite. In most valve applications this is to all intents and purposes 

 true, for the input resistance is certainly sufficiently higher than the source 

 or generator resistance to be neglected. However, in a few applications such 

 as micro-electrode recording in electrophysiology or pH measurement, the 

 generator (micro-electrode or pH probe) resistance is itself extremely high, 

 and the valve input resistance becomes important. 



In point of fact a small but significant grid current, which may he in the 

 range 10~^ to 10~^^ amps with ordinary radio receiving type valves, flows 

 either into or out of the valve, depending on the bias used. The current is 

 due to the net effect produced by a number of causes — electrons hitting the 



169 



