. The thermionic vacuum tube and its applications . he platecircuit the phase difference between the plate and grid potentialsmay differ from 180°. When this happens the dynamic character-istic of the plate circuit takes the form of a loop. To explainthis we can make use of the theorem stated on page 157, that avoltage eg apphed between filament and grid is equivalent to anelectromotive force iieg impressed on the plate circuit, where ixis the ampUfication constant of the tube. The phase relations are shown in Fig. 87 for various values of the angle 0 = tan~^ — of the external impedance. Let t
. The thermionic vacuum tube and its applications . he platecircuit the phase difference between the plate and grid potentialsmay differ from 180°. When this happens the dynamic character-istic of the plate circuit takes the form of a loop. To explainthis we can make use of the theorem stated on page 157, that avoltage eg apphed between filament and grid is equivalent to anelectromotive force iieg impressed on the plate circuit, where ixis the ampUfication constant of the tube. The phase relations are shown in Fig. 87 for various values of the angle 0 = tan~^ — of the external impedance. Let the plate cuirent be represented THE THERMIONIC AMPLIFIER 175 by ip in the direction OQP. The voltage drop ipVp, in the tube,due to its plate resistance, is given by OQ. The drop ipZo in theexternal impedance Zq is given by Qa. Thus, in the case in whichthe angle 0 is 45°, ipZo^Qao, and is the vector sum ipVo and ipXo,the total driving , fiCg in the plate circuit is in this casegiven by Oa2. Now ep is equal to —ipZo and is given by Oc2,. Ond Vol-f-S Fig. 88. which is parallel to Qa2. The phase difference between ep andHBg or eg is therefore equal to the angle a20c2 which is °,This is for the case in which the external impedance Zq is numeri-cally equal to the plate resistance Vp (0Q = Qa2), and has an angleof 45°. Referring now to Fig. 88, let the negative grid batteiy voltage 176 THERMIONIC VACUUM TUBE be equal to MS, so that we operate around the point 0 of the tubecharacteristic AOB, which corresponds to the plate potentialwhich obtains when the alternating potential impressed on thegrid is zero. The other two tube characteristics are for the maxi-mum and minimum potentials which the plate acquires when analternating potential e sin pt is superimposed on the constantnegative grid potential Eg = MS. If we now plot the plate cur-rent as a function of the varying grid potential Cg, considering at thesame time that eg and the alternating plate potential ep are
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