Two to four wire hybrid circuit

A two- to four-wire hybrid circuit for coupling a bidirectional terminal line to a transmission medium having separate branches for a receiving and a transmitting direction of analog electrical signals, said circuit comprising:

• (a) a line coupling circuit having a pair of terminal connectors connected to said terminal line;
• and a first and a second transmission connector;
• (b) first amplifying means, including an input adapted to receive respective ones of said electrical signals from said receiving branch and a first and a second output of low impedance delivering output signals in phase opposition;
• said first output being connected to said first transmission connector;
• (c) a first impedance designed as plug-in unit and arranged between said second output and said second transmission connector and having a value of the coupled terminal line impedance, effective at said transmission connectors;
• (d) second amplifying means having a first and a second input both of high impedance and an output adapted to transmit respective ones of said electrical signals to said transmitting branch;
• said first input of said second amplifying means being connected to said second transmission connector;
• (e) a series circuit including an attenuator and a second impedance designed as plug-in unit, both connected via a connector tap, said series circuit being arranged between said outputs of said first amplifying means, said connector tap being connected to said second input of said second amplifying means wherein the values of said series circuit are chosen such that the resistance value of said attenuator is substantially higher than the value of the actual resistance component of the first impedance and the ratio of said coupled line impedance to said first impedance equals the ratio of the resistance of said attenuator to said second impedance.