Subscriber line circuit having improved AC impedance matching

Claim 1

In a subscriber line circuit having a tip conductor and a ring conductor connected to a subscriber loop circuit and to a source of AC signals, an improved AC impedance matching arrangement comprising: a first amplifier means including a first output connected to said tip conductor through first resistance means having a set DC value and a second output connected to said ring conductor through second resistance means having a set DC value, said first amplifier means arranged to supply loop current to said subscriber loop circuit, said first amplifier further including a first input and a second input; second amplifier means including a first input connected to said tip conductor and a second input connected to said ring conductor, said second amplifier means arranged to develop an output signal responsive to AC signals applied to said tip and ring conductors from said source of AC signals; feedback generating means connected to said second amplifier means, said feedback generating means arranged to receive said second amplifier means output signal and output a feedback signal; and, first selection means including an input connected to said feedback generating means and first and second outputs connected to said first amplifier means first and second inputs respectively, said first selection means further including an enabling lead connected to common control equipment and a source of enabling signals, said first selection means arranged to connect said first amplifier first input or alternatively said second input to said feedback generating means responsive to said enabling signals; and said feedback signal is passed to said first amplifier means raising the impedance seen by said tip and ring conductors above the DC value of said first and second resistance means.