522 BELL SYSTEM TECHNICAL JOURNAL 



circuit coupled to the output of the amplifier, at the junction A, by 

 a high series resistance, Rf, and to the input of the amplifier, at the 

 junction B, through a winding coupled directly to the inductance. 



The amplifier is designed to have ample load carrying capacity 

 so that its gain varies but little with changes in load. This, as we 

 have seen, is necessary in order to make the amplitude equilibrium 

 curve steep and the frequency less subject to variation through 

 unavoidable changes in phase displacement. Moreover, the voltage 

 which appears at the junction /I, as a result of a gi\'en N'ohage im- 

 pressed upon the junction B, is stabilized by making the sum of the 

 resistances R\ and R-i low as compared with the load impedance and 

 with the impedance of the frequency control unit. In particular, i?2, 

 across which I he load is connected, is so small in comparison with 

 the impedance of the load that changes in the latter are entirely 

 negligible. Such an arrangement does not, of course, lead to high 

 efficiency, but we must be prepared to make concessions in one direction 

 in order to secure benefits in another. By making the effecti\e load 

 applied to the tube largely resistance the phase displacement occurring 

 in the amplifier is made very small. 



In this circuit it is not necessary to use two tubes to obtain the 

 proper |)hase relations. .-\t resonance the apparent impedance of 

 tile siiunt resonant circuit apj^roaches a pure resistance and the 

 \()ltage drop across it is conseciuenth' in phase with the e.m.f. acting 

 in the plate circuit of the tube. The current through the inductance, 

 however, lags 00° behind this \-o!tage. The e.m.f. set up in the 

 oscillator input winding is !)()° out of phase with the current in the 

 priinar\', thus making it possible to secure in the frcfiuency control 

 iniii till' necessary 180° phase reversal between the plate and grid 

 circuits of the tube. Care must, of course, be exercised to connect 

 the windings of the oscillating coil in the proper direction. 



The damping nl the resonant circuit may be made small b\' gixiiig 

 the primar\' wiiidini.; of tlie oscillating coil a high time constant. The 

 coupling impedances introduce additional damping which ma>' be 

 made small by making both the feed-back resistance, /?/, and the 

 input inipi(!an(c of the tube high. The tube imjiedance may be 

 made \ cr\ lii);ii li\ using sufficient negati\'e grid bias to pre\'ent the 

 filament-grid circuit from becoming conductive. I >f ilu- two cmiijliiig 

 impedances, the feed-back resistance, together witli ilu' oilur im- 

 pedances associated wiili ilic iiilie (uitpiii. introduces the greater 

 damping. Now it can ln' >ht)\\n thai Inr tlie frequency- control unit 

 to ha\e a given transmission efticiencN-, the total adilcd daniiiing due 

 to coupling is a minimum when the damping due to ihc input coupling 



