Subscriber line interface circuit having means for combining DC and AC feedback signals

Claim 1

A subscriber line interface circuit comprising: a balanced amplifier having a positive input terminal and a negative input terminal for amplifying a voltage across said positive and negative input terminals and generating a pair of mutually complementary voltages across a true output terminal and a complementary output terminal, the complementary voltages varying oppositely to each other and symmetrically with respect to a reference voltage; four resistors of a first, substantially equal resistance value respectively connected between a first terminal of a subscriber loop and said negative input terminal, between a second terminal of the loop and said positive input terminal, between said negative input and true output terminals and between said positive input and complementary output terminals; two resistors of a second, equal resistance value respectively connected between said first terminal of the loop and said complementary output terminal and between said second terminal of the loop and said true output terminal, said second resistance value being smaller than a reference impedance of said subscriber loop; a DC feedback circuit connected to one of said true and complementary output terminals of the amplifier for generating a DC feedback signal for providing a desired variable DC power feeding characteristic; an AC feedback circuit connected to said first and second terminals of the loop for generating an AC feedback signal which is proportional to a differential voltage across said first and second terminals and inversely proportional to a desired AC impedance; and a combining circuit for combining said DC feedback signal and said AC feedback signal, deriving a current from the combined signals and supplying the current to one of said positive and negative input terminals of the amplifier.