Limits to Amplification

O I S E in amplifiers is now a familiar term. A n y one who has had his favorite radio hour ruined by static knows the effect of an incoherent background of noise. Although static was one of the first noises observed in radio amplifiers, its origin is really outside the circuits. At one time it seemed that other sources of noise of purely local origin, such as poor batteries, loose contacts, gassy tubes and induction from power lines, might be eliminated entirely so t h a t the circuits would be capable of amplifying any signal, no matter how small. It was found, however, that the noise level cannot be lowered indefinitely; that there are limits below which, in the nature of things, noise cannot be reduced. Of the sources of noise, the most fundamental and inevitable is thermal agitation of electricity. In a perfect amplifier all other noises would be reduced to a level below that of thermal agitation. Next in order comes the influence of ions and of shot effect and flicker effect on the current in the vacuum tubes. Under control to a greater extent, b u t nevertheless of a malignant nature, are the effects of poor contacts, mechanical vibration, and h u m from a.-c. cathode heating. In dealing with these disturbances, the circuit and vacuum tube of the first stage of the amplifier are the most important, for here the signal being amplified is at its lowest level. W h e n the signal is so faint that it is masked by the noise remaining as the natural limit of the circuit, then the only possible remedy is to raise the signal level.