. The thermionic vacuum tube and its applications . -Eg-]-ej it wiW be found that as this voltage is increased by increasing either Ep or Eg, thedetecting current at first increases, reaches a maximum, and thendecreases. It is assumed that the grid is at all times negativewith respect to the negative end of the filament. Now thedetection coeffiicient a is given by the second derivative of the char-acteristic, and is a measure of the detecting current, that is, theaudio frequency component in the output. The maximum of. detecting current such as shown in Fig. 196 is due to the potentialdrop in
. The thermionic vacuum tube and its applications . -Eg-]-ej it wiW be found that as this voltage is increased by increasing either Ep or Eg, thedetecting current at first increases, reaches a maximum, and thendecreases. It is assumed that the grid is at all times negativewith respect to the negative end of the filament. Now thedetection coeffiicient a is given by the second derivative of the char-acteristic, and is a measure of the detecting current, that is, theaudio frequency component in the output. The maximum of. detecting current such as shown in Fig. 196 is due to the potentialdrop in the filament due to the heating current. It can be ac-counted for if we take regard of the voltage drop in the filament ingiving an expression for the current as a function of the plate orgrid voltage. It was shown in Section 28 that if tliis be con-sidered, the characteristic of the tube can be expressed by meansof two equations, one which holds for values of the applied platepotential less than the potential drop in the filament and the otherfor larger values of the plate potential. These two equations aregiven as equations (17) and (19) of Chapter IV. They werederived for the case of a simple valve containing only anode and 330 THERMIONIC VACUUM TUBE cathode. But we can, to a first approximation, apply the con-siderations given there to the three electrode device if we replace the plate potential b}- the expression Ey= {—-{-Eg-\-e] so that we can write the characteristic equations in the form Ip^Klf^iovE^^
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