. The thermionic vacuum tube and its applications . 182 THERMIONIC VACUUM TUBE influence the voltage amplification. For example, it followsdirectly from equation (26) that the external impedance shouldbe made several times as large as the plate resistance of the , for the same amount of filament surface the plate resistanceincreases approximately as the square of n (see equation 15)and may acquire such a high value as to necessitate an impracti-cably high external impedance. It is, therefore, often necessary,when increasing /x, to increase the amount of filament surface soas to reduce
. The thermionic vacuum tube and its applications . 182 THERMIONIC VACUUM TUBE influence the voltage amplification. For example, it followsdirectly from equation (26) that the external impedance shouldbe made several times as large as the plate resistance of the , for the same amount of filament surface the plate resistanceincreases approximately as the square of n (see equation 15)and may acquire such a high value as to necessitate an impracti-cably high external impedance. It is, therefore, often necessary,when increasing /x, to increase the amount of filament surface soas to reduce the plate resistance as much as possible. It is, ofcourse, also possible to decrease the plate resistance by increasingthe d-c. plate voltage, provided we do not operate beyond theminimum saturation voltage. Referring now to equation (26) let Zo = rQ-\-jxo; the voltageamplification is then given by M-^o V(rp+ro)-+X(2 (27) Suppose the tube is inserted in the circuit shown in Fig. 90,and that it is desired to obtain the voltage developed between. Fig. 90. the ends A and B of the impedance Zq. This voltage fo can bemeasured by connecting an electrostatic voltmeter between Aand B} The secondary of the transformer T can be wound tohave as high an impedance as possible, thus impressing the highestpossible voltage Sg on the grid for a given voltage in the primaryof r. Let us now consider the two extreme cases in which Zq is (1)a non-inductive resistance ro {xo^O) and (2) a practically pure 1 A thermionic tube can be used as an electrostatic voltmeter in the mannershown in Section 114. THE THERMIONIC AMPLIFIER 183 reactance xq (ro=0). In the first case the voltage amphficationis given by eg Tp-\-rQ (28) The relation between — and — is shown by curve II of Fig. 91 Cg Tp from which it is seen that — reacnes about 90 per cent of its eg maxunum value ^u when ro= 10 Vp. (In computing these curves u was taken equal to 10.) 864 / /V ^ // /II // // \ } I \ i I 3 1 Fig. 91. If Zo is a pur
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