| 摘要 |
A variable impedance circuit includes a differential circuit, an impedance element, a variable current attenuator and a pair of level shifters. The differential circuit includes a transistor pair with a pair of input terminals, a pair of output terminals, and a pair of emitters. The impedance element is connected between the emitters of the transistor pair. The variable current attenuator has a pair of input terminals connected respectively to the output terminals of the transistor pair, a pair of output terminals, and a pair of control inputs for receiving a control voltage to vary the current gain of the variable current attenuator. Each of the level shifters has an input terminal connected to a respective one of the output terminals of the variable current attenuator, and an output terminal connected to a respective one of the input terminals of the transistor pair. The level shifters ensure that the transistor pair and the variable current attenuator are properly biased. When the current gain of the variable current attenuator is varied, an impedance measured across the output terminals of the variable current attenuator is correspondingly varied.
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