Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 CATHODE FOLLOWER CATHODE FOLLOWER When i?^ = 0 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 ). If we have the same valve as before and R^ = 10,000 Q, Rj^ _ 10,000 _ '^^^ ^ ^a + Rk ~ + 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 chang
Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 CATHODE FOLLOWER CATHODE FOLLOWER When i?^ = 0 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 ). If we have the same valve as before and R^ = 10,000 Q, Rj^ _ 10,000 _ '^^^ ^ ^a + Rk ~ + 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 cau
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