. The thermionic vacuum tube and its applications . high that the effective grid voltage -+£-.+ 6 is not high enough to draw all the electrons to the anode as fast as they are emitted fromthe cathode, for if this were so, a change in the applied voltagewould not produce any appreciable change in the anode other words, the temperature of the filament must be highenough to comply with the condition of temperature saturation THE THERMIONIC AMPLIFIER 225 which is indicated by the horizontal part of the plate cun-cnt-filanient current characteristic (see Fig. 18, page 51). The effectof f
. The thermionic vacuum tube and its applications . high that the effective grid voltage -+£-.+ 6 is not high enough to draw all the electrons to the anode as fast as they are emitted fromthe cathode, for if this were so, a change in the applied voltagewould not produce any appreciable change in the anode other words, the temperature of the filament must be highenough to comply with the condition of temperature saturation THE THERMIONIC AMPLIFIER 225 which is indicated by the horizontal part of the plate cun-cnt-filanient current characteristic (see Fig. 18, page 51). The effectof filament temperature is indicated in Fig. 117, which gives therelation between the filament voltage and the amphfication meas-ured at a constant plate voltage in a circuit like that shown inFig. HI. For this particular type of tube, it is seen from thecurve, the filament voltage must never drop below volts. Onthe other hand, it should not be increased more than is absolutelynecessary, for this would shorten the life of the filament. Satisfac-. FiG. 116. tory operation and long Hfe can therefore be obtained by suitablyadjusting the filament current. Referring to equation (36) it is evident that the amplification increases as the ratio — is increased provided the plate resistance Tp always remains equal to the external impedance of the platecircuit. For a given type of tube m remains practically constant,but Tp can be decreased by increasing the plate potential providedthe filament temperature remains high enough to insure tempera-ture saturation. (It will l^e remembered that the filamenttemperature necessary for this condition increases as the platevoltage is increased.) It is therefore to be expected that the 226 THERMIONIC VACUUM TUBE amplification would increase with increase in the plate the anipUfication is measured for increasmg plate voltages Epin the circuit of Fig. Ill, which contains a fixed external impedance,the amplification generally tends toward a
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