Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 NEGATIVE VOLTAGE FEEDBACK AND THE STABILIZED GAIN AMPLIFIER grid, ions hitting the grid, leakage of current through surface moisture on the valve envelope, and the whole lot combine to produce a curve having the general shape of Figure The slope of the curve at any particular Into valve ii Grid current, Ig ii 1 Grid bias, Vgi^ J Q 1 ^ Negative p bias /^ '*'--«i^ Positive bias HT+ Out of valve Figure Figure value of bias gives the value of the inpu
Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 NEGATIVE VOLTAGE FEEDBACK AND THE STABILIZED GAIN AMPLIFIER grid, ions hitting the grid, leakage of current through surface moisture on the valve envelope, and the whole lot combine to produce a curve having the general shape of Figure The slope of the curve at any particular Into valve ii Grid current, Ig ii 1 Grid bias, Vgi^ J Q 1 ^ Negative p bias /^ '*'--«i^ Positive bias HT+ Out of valve Figure Figure value of bias gives the value of the input resistance. The input resistance is positive between Q and R, infinite at Q and negative between P and Q. What does negative resistance mean? The power absorbed by a positive resistance is V'^fR, so the power absorbed by a negative resistance is — V^jR, that is, it does not absorb power, it generates it. We cannot have spurious generators associated with the input circuits of measuring devices, and the net input resistance must somehow be made positive. The spurious generator is most powerful when the negative resistance is lowest. If this value is —-Rj-min, then if we connect between grid and cathode an ordinary resis- tance Rg, the resistance of the parallel combination is Rg — Ri and this is positive if Rg < R^. This is the explanation for the maximum value of grid resistance permis- sible in amplifying stages being stated by the valve manufacturers. It means that the valve can be operated over the bias range corresponding to the PQ section of the grid current characteristic without instability. Over the QR range there is no difficulty anyway, as the input resistance is positive. Now let us see how this affects the cathode follower. Suppose we are working in the region where the input resistance is positive. If we make the grid positive, say, an amount dVg, then in an ordinary amplifying stage the change in grid current is dig and the input resistance is R^
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