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A plate detector (or anode-bend detector) is a vacuum tube detector circuit used in A.M. radios, although the term is sometimes extended to transistor circuits that similarly rely upon distorted amplification to enhance one half of the received signal over the other. This circuit employs a tube with an indirectly heated cathode, typically a medium-mu triode, or a tetrode or pentode with a sharp cut-off control grid. Partial rectification of R.F. signals occurs in the plate of the detector tube if it is forced into its non-linear region at roughly the zero crossing point of the input signal. This differs from a grid-leak detector, which achieves rectification in the control grid. It also differs from the diode detector circuit commonly used to provide both R.F. rectification and automatic volume control (A.V.C.) bias to the R.F. amplifier tubes. == Overview == Enough negative bias is applied to the grid (or ''base'', for a transistor implementation) to bring the device almost to cut-off point; an incoming signal will cause the device to conduct only on positive-going halves of the input signal... it cannot conduct significantly ''less'' during negative halves, so the plate voltage varies at the modulation frequency - going more negative as the input signal envelope increases with the Amplitude Modulation modulating signal. Plate detectors are used in both T.R.F. and (less commonly) superheterodyne receivers. The grid is connected directly to the secondary of the final R.F. or I.F. transformer. The cathode is connected to ground through a circuit consisting of a parallel-connected bias resistor (usually 10 kΩ to 50 kΩ) and bypass capacitor (usually 0.25 µF to 0.5 µF). When sufficient negative bias is applied to the grid, the plate current is pushed almost to the cut-off point. When a modulated R.F. signal is applied to the grid under these conditions, a corresponding increase in plate current occurs. As the signal amplitude varies, the plate current also varies, causing the plate to act as a diode detector while the tube as a whole also acts as an audio amplifier. A plate bypass capacitor (usually 500 pF to 0.002 µF for triodes, or 250 pF to 0.001 µF for tetrodes and pentodes) is used to regulate plate current. Like most A.F. amplifiers in radios, the plate voltage is usually less than 60 volts. When a tetrode or pentode is used, the screen grid voltage is usually about one-half the plate voltage. Plate detector circuits were commonly used from the introduction of indirectly heated cathode tubes in the late 1920s until the start of World War II. As R.F. tubes became more sensitive, grid-leak detectors (which are more sensitive than plate detectors) became less practical. Diode detectors were popular because, unlike plate detector circuits, they could also provide A.V.C. bias. However, the dual-diode/triode and dual-diode/pentode tubes commonly used in these circuits had bulk wholesale costs that were as much as twice the cost of the tubes commonly used as plate detectors. This made plate detector circuits more practical for low-priced radios sold during the depths of the Great Depression. Because an indirectly heated cathode is required (or a separate negative grid bias battery is needed) for this circuit to operate, it is not used in battery-operated radios. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Plate detector (radio)」の詳細全文を読む スポンサード リンク
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