. The thermionic vacuum tube and its applications . Fig. 169. Fig. 170 shows a Colpitts circuit as it can be used in tliis case the resistance Rs is replaced by a choke coil alternating and direct current in the plate circuit are sep-arated by means of the choke coil C/i2 and the capacity Cs. The. ^^m^msm3 Fig. 170. inductance of this choke coil is usually chosen as high as possible,or at least so high that its impedance is several times the plateresistance of the tube. The capacities Cs and Cb are chosensufficiently large so that they do not appreciably affect the opera-ti
. The thermionic vacuum tube and its applications . Fig. 169. Fig. 170 shows a Colpitts circuit as it can be used in tliis case the resistance Rs is replaced by a choke coil alternating and direct current in the plate circuit are sep-arated by means of the choke coil C/i2 and the capacity Cs. The. ^^m^msm3 Fig. 170. inductance of this choke coil is usually chosen as high as possible,or at least so high that its impedance is several times the plateresistance of the tube. The capacities Cs and Cb are chosensufficiently large so that they do not appreciably affect the opera-tion of the oscillation circuit LC1C2. VACUUM TUBE AS AN OSCILLATION GENERATOR 295 91. Efifect of Grid Current. In deriving the conditions foroscillation above, it was assumed that the grid is at all timesmaintained sufficiently negative with respect to the filament toprevent any convection current from flowing between filament andgiid. In practice this is usually not the case. The grid usuallybecomes positive to an extent depending on the adjustments ofthe circuit constants. Thus, when using the condenser andresistance to maintain the grid negative, as shown in Fig. 169,the grid must become positive during part of the cycle. Therectified current through Rs maintains the grid at a steady nega-tive potential, an
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