fundamental
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The change of output voltage is related to the change of voltage Ube between the triode base and the emitter, and through the change of Ube, the internal resistance of the tube and the current of the tube are affected, so as to ensure that the output voltage is basically unchanged.
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For example, when the output voltage increases, the Ube decreases, so that the equivalent resistance between the triode collector and the emitter increases, and the collector current decreases, so that the output voltage remains unchanged.
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On the contrary, when the output voltage decreases, the Ube increases, so that the equivalent resistance between the triode collector and the emitter decreases, and the collector current increases, so that the output voltage remains unchanged.