510 



BELL SYSTEM TECHNICAL JOURNAL 



point .1 Id ilif point B. through the frequency- control unit. Due 

 account must he taken of any energy delivered to the load circuit. 

 In other words, when a gi\en amount of energy flows into the ampli- 

 fier across the junction B. it must be transmitted annmd the regener- 

 ative loop and returned to this junction unchanged in amount. The 

 second condition is that the phase displacement of the wave trans- 

 mitted from A to B through the frequency control unit must be equal 

 in amount and opposite in sign to the phase displacement of the 

 wave transmitted fiom .6 to ^ through the amplifier. That is, a 

 wa\e which enters the amplifier at the junction B must be trans- 

 mitted through the regenerative system and returned to this junction 

 with no resultant phase displacement.' The individual character- 

 istics of the amplifier and of the frequency control unit which permit 

 these two conditions to be satisfied fix the operating point of the system. 

 Although the reasoning to be used in the succeeding paragraphs 

 may, in general, be applicfi to any oscillatory system, it will be easier 

 to follow if descriijcd in terms of familiar electrical circuits. In Fig. 2 



Fig. 2 — Circiiil of :iTi L-lcim-iU.ir\ 



the ami)lilier and associati-d liatteries are shown in the lorni of an 

 elementary \acuum tube cirruii. It is necessary to use a two-stage 

 circuit if the voltages at ,1 and at B are to ha\'e the same sign. This 

 is because the grid voltage, which is obtained as a potential drop due 

 to current from an external source flowing through a resistance con- 

 nected between the grid and the filament of the vacuum tube, reduces 

 the current flowing from the filament of the tube to the plate, through 

 a second external resistance, as the current flowing from the grid to the 

 filament is reduced. That is, a change in the \oltage drop across 



'This condition for stable equililuiuin will ,ilsi) In 

 shift around the loop is cfiiial to 2ir»; wlurc ii iii.n In- .ii 



^.itislicil il llu- loi.il ph.isf 

 whole niinihcr. 



