576 BELL SYSTEM TECHNICAL JOURNAL 



so that the attenuation through the bridge is just equal to the gain of 

 the ampUfier. 



It is evident that if all the bridge arms had fixed values of resistance, 

 the attenuation of the bridge would be very critical with slight changes 

 in any arm. This would obviously be undesirable, for the circuit 

 would either fail to oscillate, or else build up in amplitude until tube 

 overloading occurred. The thermally controlled resistance Ri elimi- 

 nates this difficulty. This arm has a large positive temperature coeffi- 

 cient of resistance, and is so designed that the portion of the amplifi.er 

 output which reaches it in the bridge circuit is great enough to raise its 

 temperature and increase its resistance materially. A small tungsten- 

 filament lamp of low wattage rating has been found suitable. It 

 functions as follows: 



When battery is first applied to the amplifier, the lamp Ri is cold and 

 its resistance is considerably smaller than the balance value. Thus 

 the attenuation of the bridge is relatively small, and oscillation builds 

 up rapidly. As the lamp filament warms, its resistance approaches 

 the value for which the loss through the bridge equals the gain of the 

 amplifier. If for some reason Ri acquires too large a resistance, the 

 unbalance potential e becomes too small or possibly even inverted in 

 phase, so that the amplitude decreases until the proper equilibrium is 

 reached. 



This automatic adjustment stabilizes the amplitude, for the amount 

 of power needed to give Ri a value closely approaching {RiRzjjRi is 

 always very nearly the same. A change in the amplifier gain would 

 cause a readjustment of the bridge balance, but the resulting variation 

 in R\ or in the amplifier output would be extremely small. The 

 operating temperature of the lamp filament is made high enough so 

 that variations in the ambient temperature do not affect the adjust- 

 ment appreciably. 



No overloading occurs in the amplifier, which operates on a strictly 

 Class A basis, nor is any non-linearity necessary in the system other 

 than the thermal non-linearity of Ri. As the lamp resistance does not 

 vary appreciably during a high-frequency cycle, it is not a source of 

 harmonics (or of their intermodulation, which Llewellyn ^ has shown 

 to be one of the factors contributing to frequency instability). 



In contrast to the lamp, an ordinary non-linear resistance, of copper 

 oxide for example, would not be suitable for Ri. A resistance of the 

 thermally-controlled type having a negative temperature coefficient 



1" Constant Frequency Oscillators," F. B. Llewellyn, Proc. I. R. E., December 

 1931. 



