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Tetrode
A tetrode is a vacuum tube having four active electrodes. There are several varieties of tetrode with screen-grid tube, and the beam tetrode being the most common. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate. In screen-grid tubes and beam tetrodes, the first grid is the control grid and the second grid is the screen grid.〔L.W. Turner, (ed), ''Electronics Engineer's Reference Book'', 4th ed. Newnes-Butterworth, London 1976 ISBN 0408001682 page 7-19〕 In other tetrodes one of the grids is a control grid, while the other may have a variety of functions.
During the period 1913 to 1927, three distinct types of tetrode valves appeared. All had a normal control grid whose function was to act as a primary control for current passing through the tube, but they differed according to the intended function of the other grid. In order of historical appearance these are: the space-charge grid tube, the bi-grid valve, and the screen-grid tube. The last of these appeared in two distinct variants with different areas of application: the screen-grid valve proper, which was used for medium-frequency, small signal amplification, and the beam tetrode which appeared later, and was used for audio or radio-frequency power amplification. The former was quickly superseded by the rf pentode, while the latter was initially developed as an alternative to the pentode as an audio power amplifying device. The beam tetrode was also developed as a high power radio transmitting tube, and has remained in use until quite recently in both applications.
==How it works==
The tetrode functions in a similar way to the triode, from which it was developed. A current through the heater or filament heats the cathode, which causes it to emit electrons by thermionic emission. A positive voltage is applied between the plate and cathode, causing a flow of electrons from the cathode to plate through the two grids. A varying voltage applied to the control grid can control this current, causing variations in the plate current. With a resistive or other load in the plate circuit, the varying current will result in a varying voltage at the plate. With proper biasing, this voltage will be an amplified (but inverted) version of the AC voltage applied to the control grid, thus the tetrode can provide voltage gain. In the tetrode, the function of the other grid varies according to the type of tetrode; this is discussed below.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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