Interface circuit with impedance adaptation means

An interface circuit between a bidirectional telephone line carrying in particular low frequency voice signals, and a telephone switching network connected to said interface circuit via a first and a second unidirectional lines, said interface circuit including means for adjusting the impedance (Z.sub.T) seen by said telephone line and including:

• a high frequency transformer including a primary with two end terminals and a center tap K, and a secondary;
• means for connecting said secondary to said bidirectional line through a rectifier circuit;
• switching means for respectively connecting the end terminals of said primary to first and second switches which alternately open and close at a predetermined frequency;
• a load of impedance Z.sub.s connecting said switches to ground;
• first amplification means whose predetermined gain A1(.omega.) is a function of frequency and which is connected to said load;
• addition means connected to said first amplification means and to said first unidirectional line;
• second amplification means whose predetermined gain A2(.omega.) is a function of frequency and which has an input from said addition means and an output to said center tap;
• and
• means for connecting said center tap to said second unidirectional line wherein said gains A1(.omega.) and A2(.omega.) are made adjustable so that the product A1(.omega..A2(.omega.) will be substantially proportional to Z.sub.T in a predetermined frequency range.