Limits to amplification

The amplification obtainable in a vacuum tube amplifier is limited by the noise in the circuit. Of the various sources of noise the most fundamental and inevitable is thermal agitation of electricity. Other sources are the influence of ions and of shot effect and flicker effect on the current in vacuum tubes, poor contacts, mechanical vibration, and hum from a-c cathode heating. These noises and their effects in limiting amplification are discussed in this paper. Although the natural noise level of an amplifier is exceedingly low, modern amplifiers have reached such a stage of perfection that their noise levels often are practically at the natural limit.