Abstract: In one embodiment, a filter with a main signal path having one or more biquadratic filter sections is tuned using a tuning circuit based on a biquadratic filter that can be configured to oscillate at the filter's cutoff frequency. In one application, a tuning circuit outside of the main signal path is used to tune each biquadratic filter section of the main signal path. In another application, each filter section along the main signal path has a biquadratic filter that can oscillate and corresponding tuning elements that enable the filter section to tune itself. According to certain embodiments of the present invention, a biquadratic filter is made to oscillate by applying a common-mode voltage signal to the inputs of the filter's third transconductor cell to make the cell's transconductance go to zero. The invention may also be implemented in the context of filters having ladder structures in their main signal path.

Abstract: A capacitor cancellation method and apparatus for use in an interface circuit having a transformer blocking capacitor. The method includes sensing a voltage across the transformer blocking capacitor and generating a cancellation signal to compensate for the effect of the transformer blocking capacitor. The apparatus includes a sensor to sense a differential voltage across the transformer blocking capacitor to develop a capacitor signal and an amplifier to amplify the capacitor signal to obtain a cancellation signal.

Abstract: An amplifier having an improved output current drive capability includes an input stage and an output stage. An input of the output stage is operatively coupled to an output of the input stage. The amplifier further includes a current regeneration circuit operatively coupled to the input of the output stage in a feedback arrangement, the current regeneration circuit feeding back a current to the output circuit in accordance with a predetermined scale factor, the fed back current being proportional to an input current supplied to the output stage. The input current supplied to the output stage is dynamically adjustable by the current regeneration circuit in response to an input current requirement at the output stage.

Abstract: A capacitor cancellation method and apparatus for use in an interface circuit having a transformer blocking capacitor. The method includes sensing a voltage across the transformer blocking capacitor and generating a cancellation signal to compensate for the effect of the transformer blocking capacitor. The apparatus includes a sensor to sense a differential voltage across the transformer blocking capacitor to develop a capacitor signal and an amplifier to amplify the capacitor signal to obtain a cancellation signal.

Abstract: An amplifier having an improved output current drive capability includes an input stage and an output stage. An input of the output stage is operatively coupled to an output of the input stage. The amplifier further includes a current regeneration circuit operatively coupled to the input of the output stage in a feedback arrangement, the current regeneration circuit feeding back a current to the output circuit in accordance with a predetermined scale factor, the fed back current being proportional to an input current supplied to the output stage. The input current supplied to the output stage is dynamically adjustable by the current regeneration circuit in response to an input current requirement at the output stage.

Abstract: A line circuit whose circuit components are selectively controllable by a microprocessor/DSP to provide normal BORSCHT functions or to be selectively tested without need for an electromechanical relay to isolate the line circuit from the loop conductors. The conventional line transformer is advantageously replaced by a loop current feed resistor network and either a high voltage or a lower voltage battery may be connected to provide loop current to the resistor network through a current regulator circuit and battery switch circuit. In normal operation, an ac feedback path, which includes the line receive amplifier, increases the terminating impedance above the dc value of the feed resistors so the loop termination resistor network may match the characteristic impedance of the loop at the audio frequencies employed.

Abstract: A line circuit provides BORSCHT functions and supplies the customer loop through a feed resistor network and current limiter either from a high voltage battery to provide for maximum signaling range or from a lower voltage battery to conserve power. The current limiter operates under the control of a microprocessor/DSP which selectively applies or denies power to isolate or include components for testing and controls the switching in and out of transistors to control the scaling factor between the channel width-to-length ratios of the current mirror's input and output FET transistors according to the type of test or operating condition dictated by the microprocessor while the input reference current to the current mirror is maintained constant.