. Electronic apparatus for biological research . Figure as the source of the signal, in series with it. V^ is thus — F^ + f^sig- A mean current flows down the load resistance and through the valve, pro- ducing a voltage drop across the load so that the anode potential is less than the HT voltage. The bias and anode voltages are chosen so that the valve operates on a linear part of the anode and grid characteristics, so that the parameters g^, r„ and ^ are as nearly as possible constant. At first sight it might appear that as dVg^ Fgig, 61 ^ = gmVsig and there- fore dV^ — RgmKis- Then the
. Electronic apparatus for biological research . Figure as the source of the signal, in series with it. V^ is thus — F^ + f^sig- A mean current flows down the load resistance and through the valve, pro- ducing a voltage drop across the load so that the anode potential is less than the HT voltage. The bias and anode voltages are chosen so that the valve operates on a linear part of the anode and grid characteristics, so that the parameters g^, r„ and ^ are as nearly as possible constant. At first sight it might appear that as dVg^ Fgig, 61 ^ = gmVsig and there- fore dV^ — RgmKis- Then the amplification would be SVjVsig = g^l^. In fact this is not so, for if V^ changes, SlJdVg ^ g,,,. In words, suppose the grid be made a certain amount less negative. The anode current increases but there is an increased voltage drop across the load and the anode becomes less positive. As a result the anode current does not increase as much as it would have done had the anode potential been fixed. The actual amphfica- tion can be found either analytically or graphically. The answer can be found in a few seconds from the analytic solution for an existing design. In creating a new design the graphical approach has to be adopted anyway because it is the quickest way of finding the required load resistance and bias. Analytic expression of amplification If then SL uJVa and /•„ = 1 'dV„ SI. aJl 137
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