| 摘要 |
A low-pass variable filter stage having a variable impedance circuit and may include an output buffer. The variable impedance circuit includes a transistor and a diode whose collector and anode respectively are connected to a current mirror and whose base and cathode respectively are connected to a voltage controlled current source. An external capacitor is connected across the diode to an external potential and the buffer is connected across the diode input. A first static voltage is applied producing a first static current in the transistor collector which is mirrored into the diode anode circuit as a second static current producing a second static voltage across the diode. The sum of the static currents form a control current flowing through the current source whose amplitude controls the small signal resistance of the diode. A small signal is superimposed upon the second static current. The diode resistance, the signal frequency, and the capacitor control which frequencies pass the filter. Cascading stages with direct feed provides component reduction and electrical noise suppression while the buffer provides impedance matching to reduce filter stage loading. The capacitor connected to the external potential releases integrated circuit pin connectors for other functions.
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