. Electronic apparatus for biological research . Figure characteristic of the valve. The mutual conductance is (dlJdVg)^^ and is given by the slopes of the curves in the grid characteristic. It is moderately constant if /„ is not too large or too small; it falls as the valve wears out. Its units are amperes per volt, but it is usually measured in milliamperes per volt. It is not necessary to plot out the grid characteristic to find g^. It can all be done from the anode characteristic—usually supplied by the valve manu- facturers—for if 11 = and fa^ then A« r^^ai Ud l^v;] L is As we have se
. Electronic apparatus for biological research . Figure characteristic of the valve. The mutual conductance is (dlJdVg)^^ and is given by the slopes of the curves in the grid characteristic. It is moderately constant if /„ is not too large or too small; it falls as the valve wears out. Its units are amperes per volt, but it is usually measured in milliamperes per volt. It is not necessary to plot out the grid characteristic to find g^. It can all be done from the anode characteristic—usually supplied by the valve manu- facturers—for if 11 = and fa^ then A« r^^ai Ud l^v;] L is As we have seen, /j, and r^ can be read off the anode characteristic; g given by their ratio. In general it is sufficiently true to say that, because the triode valve is operated with the grid biased negatively with respect to the cathode the electrons will be obliged to pass between the interstices of the grid on their way to the anode—being repelled away from the grid wires—and that therefore no current can flow into or out of the grid. This is the same as saying that the resistance 'looking in' at the grid of the valve is infinite. We say that the valve is 'voltage operated' rather than 'power operated'. SIMPLE AMPLIFICATION WITH THE TRIODE To get amphfication or 'gain' with a triode valve we cause the anode current to flow through a load resistance. This resistance may be an actual load 136
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