Tunable active filter
First Claim
1. A filter comprising a balanced amplifier having an inverting input and a noninverting input, and an inverted output and a noninverted output, wherein a pair of first and second input paths are connected to said inverting input and said noninverting input, respectively, and wherein a pair of first and second feedback paths are connected between said noninverted output and said inverting input, and between said inverted output and said noninverting input, respectively, and wherein at least one of said pair of paths comprises an electronically controlled resistance in the form of a field effect transistor in each of said first and second paths therein, and wherein at least the other of said pair of paths comprises a reactive element in each of said first and second paths therein.
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Abstract
A tunable high-pass or low-pass filter employs a balanced amplifier having electronically controlled elements in either the two input or the two feedback paths (or both). A reactive element is placed in the other of the paths. Especially low distortion results from this arrangement, allowing convenient integrated circuit implementation with a nonlinear device (e.g., MOS transistor) as a voltage-controllable element. Multiple stages can be cascaded and controlled from a reference clock or other precision source for high accuracy.
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Citations
7 Claims
- 1. A filter comprising a balanced amplifier having an inverting input and a noninverting input, and an inverted output and a noninverted output, wherein a pair of first and second input paths are connected to said inverting input and said noninverting input, respectively, and wherein a pair of first and second feedback paths are connected between said noninverted output and said inverting input, and between said inverted output and said noninverting input, respectively, and wherein at least one of said pair of paths comprises an electronically controlled resistance in the form of a field effect transistor in each of said first and second paths therein, and wherein at least the other of said pair of paths comprises a reactive element in each of said first and second paths therein.
Specification