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.
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.
The general problem of obtaining oscillations by the use of diodes with critical electron transit time is outlined, Some of the properties of a 10 em. oscillator tested experimentally are included. Extraneous losses were reduced when the oscillator was enclosed within a wave guide.T H E theory of the production of negative impedance by means of an electron discharge between two parallel planes has been known for some years.' The negative resistance appears whenever the electron transit time is approximately 1~, 2~, 3~, etc. cycles of a given high-frequency current. Using this property, Muller was able to construct tubes giving 100 em. oscillations," The operating efficiencies were quite low, and in the frequency range covered by these tubes it seems fairly conclusive that other methods of producing oscillations are more effective than the critical transit time diode. However, there is promise in the application of diode operation to much higher frequencies than those of Muller.In a diode where the electron discharge occurs between two parallel planes where one performs the function of electron emitting cathode and the other constitutes an anode biased at a positive potential, the effective impedance presented to an external source is inherently low in magnitude. This is because of the capacitance between the two planes which causes the decrease in impedance at high frequencies. For the production of oscillations, the capacitance must be combined with a resonant structure having the proper inductance to resonate at the desired frequency and having a resistance which effectively is less in magnitude than that of the electron stream. Because of the low losses thus required of the coupling or tuning circuit the properties of concentric lines and of tuned cavities offer a favorable method of attack. These structures also have the property that the impedance presented to the diode proper may be made low to match its capacitive reactance at the high frequencies desired.The two most important sources of circuit resistance are ordinary ohmic loss modified in the usual way by skin effect in the conducting 1 For numbered references see end of paper. 280
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