. Electronic apparatus for biological research . Figure across /?[. 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 fo
. Electronic apparatus for biological research . Figure across /?[. 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 ). 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
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