Circuit with feedback for controlling the impedance thereof

An impedance control circuit for transferring a first voltage signal from a signal source to a second circuit with a desired source impedance and receiving a second voltage signal from said second circuit with a desired termination impedance through an interface terminal comprising

• first impedance means connected to said interface terminal,
• means responsive to a signal current generated through said first impedance means and said interface terminal for generating a first feedback current proportional to said signal current by a first proportionality constant,
• second impedance means responsive to said feedback current for generating a first feedback voltage signal related to said feedback current and said signal current, and
• means for summing said first feedback voltage signal and a voltage signal proportional to said first voltage signal by a second proportionality constant into an output signal at an output terminal, said first impedance means connected between said summing means output terminal and said interface terminal so that the difference between said summed output signal and the voltage signal at said interface terminal generates said signal current,
• whereby the impedance of said second impedance means with respect to the impedance of said first impedance means and the values of said first and said second proportionality constants determine said desired source impedance and said desired termination impedance of said circuit.

An impedance control circuit for transferring voltage signals from a signal source to a second circuit and receiving voltage signals from said second circuit through an interfacing terminal comprising

• first means for inverting a voltage signal at said terminal,
• second means for summing said inverted terminal voltage signal and said source voltage signal and amplifying said sum by a factor determined by the ratio of impedances of a first impedance means to a second impedance means,
• third means for summing and amplified summed voltage signals from said second means and said terminal voltage signal into an output signal at an output terminal, and
• a third impedance means connected between said third means output terminal and said interfacing terminal and having an impedance substantially equal to that of said first impedance means,
• whereby the impedance of said second impedance means is substantially the impedance of said circuit.

An impedance control circuit for transferring a voltage signal from a signal source to second circuit with a desired source impedance and receiving a voltage signal from said second circuit with a desired termination impedance at an interfacing terminal comprising

• first means responsive to the voltage signal at said interfacing terminal for amplifying said terminal voltage signal by a first proportionality factor,
• second means responsive to said source voltage signal for amplifying said source voltage signal by a second proportionality factor related to said first proportionality factor,
• third means for summing said amplified terminal voltage signal and said amplified source voltage signal into an output voltage signal at an output terminal, and
• a first impedance means connected between said third means output terminal and said interfacing terminal,
• whereby said first and second proportionality factors determine the desired source impedance and the desired termination impedance of said circuit.

A subscriber line interface circuit for transferring voltage signals from a signal source to a subscriber line loop with a desired source impedance and receiving voltage signals from said subscriber line loop with a desired termination impedance, comprising

• a pair of terminals for connection to said subscriber line loop,
• a pair of first impedance means each having first and second terminals, each first terminal respectively connected to one of said pair of terminals,
• a pair of feedback means each respectively responsive to a first current flowing through each one of said pair of first impedance means for generating first and second feedback currents proportional to said first current by a first proportionality constant,
• second impedance means responsive to said first and second feedback currents for generating a first feedback voltage signal proportional to the sum of said first and second feedback currents,
• first amplifier means for amplifying said first feedback voltage signal by a first predetermined factor to generate a second feedback voltage signal,
• means for summing a voltage signal proportional to said source voltage signal by a second proportionality constant, said first feedback voltage signal and said second feedback voltage signal,
• means for transferring said summed voltage signals to a second terminal of one of said pair of first impedance means,
• means for generating the negative of said summed voltage signals and transferring the negative of said summed voltage signals to a second end of the other of said pair of first impedance means to generate said first current,
• whereby the impedance of said second impedance means, the impedance of said pair of first impedance means, the values of said first and second proportionality constants and said first factor in the form of a mathematical product determine said desired source impedance and said desired termination impedance of said circuit.

A subscriber line interface circuit for transferring voltage signals from a signal source to a subscriber line loop with a desired source impedance and receiving voltage signals from said subscriber line loop with a desired termination impedance, with a pair of external series resistors between said subscriber line interface circuit and said subscriber loop, comprising

• a pair of terminals for connection to said subscriber line loop through said pair of external series limiting resistors,
• a pair of first impedance means each having first and second terminals, each first terminal respectively connected to one of said pair of terminals,
• a pair of feedback means each respectively responsive to a first current flowing through each one of said pair of first impedance means for generating first and second feedback currents proportional to said first current by a first porportionality constant,
• second impedance means responsive to of said first and second feedback currents for generating a first feedback voltage signal,
• first amplifier means for amplifying said first feedback voltage signal by a first predetermined factor related to said desired impedance to generate a second feedback voltage signal,
• second amplifier means for amplifying said first feedback voltage signal by a second predetermined factor related to the values of said external series resistors to generate a third feedback voltage signal.
• means for summing a voltage signal proportional to said source voltage signal by a second proportionality constant, said second feedback voltage signal, and said third feedback voltage sginal into a summed output voltage signal at a first output terminal connected to a second terminal of one of said pair of first impedance means,
• means responsive to a signal from said summing means for generating the negative of said summed voltage signal at a second output terminal connected to a second terminal of the other of said pair of first impedance means,
• whereby the impedance of said second impedance means, impedance of said pair of first impedance means, said first and second proportionality constants, said first and second factors in the form of a mathematical product determine said desired source impedance and said desired termination impedance of said circuit.

A subscriber line interface circuit for transferring voltage signals from a signal source to subscriber line loop with a desired source impedance and receiving voltage signals from said subscriber line loop with a desired termination impedance, comprising

• a pair of terminals for connection to said subscriber line loop,
• a pair of first impedance means each having first and second terminals, each first terminal respectively connected to one of said pair of terminals,
• first amplifier means responsive to the voltage signals at said pair of terminals for generating a differential voltage signal, second amplifier means responsive to said differential voltage signal for generating a first feedback voltage signal proportional to said differential voltage signal by a first predetermined constant,
• third amplifier means responsive to said differential volage signal and said source voltage signal for generating a second feedback voltage signal proportional by a second predetermined constant to the sum of said source voltage signal and a voltage signal proportional by a third predetermined constant to said differential voltage signal,
• means for summing said first feedback voltage signal and said second feedback voltage signal into a summed output signal at a first output terminal connected to a second terminal of one of said pair of impedance means, and
• means responsive to a summed output signal from said summing means for generating the negative of said summed voltage signal at a second output terminal connected to a second terminal of the other of said pair of first impedance means,
• whereby the impedance of said first impedance means, and first, second and third proportionality constants in the form of a mathematical product determine said desired source and said desired termination impedances.