Tube Noise

Vacuum tubes act as sources of noise because of the electronic nature of their operation. Noise arises from the fact that the current consists of the movement of discrete particles rather than the flow of a continuous fluid, causing the shot effect. If the emitting material is irregular in its nature, there may also be large low-frequency variations resulting from chunks of the emitting material varying in their emission, causing the flicker effect. Noise will also arise from variations in the secondary emission, from ionization within the tube, from emission of positive ions, reflection of electrons, from a virtual cathode, and from random variations of the division of current between elements in multielectrode tubes. Of all the above-mentioned effects, the largest and most important source of noise is the shot effect. The other sources produce noise that is usually low compared to that from the shot effect alone. In particular, noise from ionization may generally be ignored when the grid gas current is less than a few hundredths of a microampere.

Shot effect is noise due to the fact that electrons are discrete particles emitted from the cathode in a random way, so that any current resulting from such emission has a random or statistical variation that is termed “noise”. The energy corresponding to these variations is proportional to the magnitude of the average or d. c. current. The noise energy is distributed evenly over the frequency spectrum. The shot noise in a tube depends upon the magnitude of the current, as well as upon the conditions under which this current flows. The noise for a given current is maximum when the plate is absorbing all the electrons that are liberated by the cathode. If the plate does not receive all the electrons emitted by the cathode, then the noise is much less because of a cushioning effect upon the variations in the rate of emission produced by the great number of electrons emitted.

3. Choose the proper word from the brackets.

1. When the cathode is heated, it (emission, to emit, emitter, emits) electrons. 2. The materials that have very few or no free electrons are called (to insulate, insulation, insulators). 3. This method is based on the ability of a material (to conduct, conduction, conducts, conductor) electricity. 4. The electrons are (repelled, attracted, attracting) by the positive voltage on the plate. 5. The shot noise in a tube (responds to, increases, depends upon, decreases) the magnitude of the current. 6. If a negative voltage (application, is applied, to apply, are applied) to the plate, current does not flow.

Дата добавления: 2015-07-18