Hybrid circuit in a telephone subscriber interface circuit

Claim 1

A hybrid circuit for a telephone subscriber interface circuit comprising: a pair of input terminals (3, 4) connected to a subscriber line (2), a DC power source (33) for supplying speech current to a telephone set (1) through said subscriber line (2), a pair of current amplifiers (17a, 17b) the outputs of which are connected to said subscriber input terminals (3, 4), respectively, for supplying DC current and speech signal to said telephone set (1), first inputs of said current amplifiers (17a, 17b) being coupled with ground and said power source (33), respectively, and second inputs of which are coupled with a current source (18) through a plurality of nodes, so that the current from the power source (33) to the subscriber line is adjusted by the control current in said second inputs, a resistor network (19) coupled with said subscriber input terminals (3, 4), a pair of DC monitor circuits (20a, 20b) for adjusting the speech current from said current amplifiers (17a, 17b) by supplying control current to a first pair of nodes (a, a') in the path of said second inputs of said current amplifiers (17a, 17b), according to the voltage across the output of said resistor network (19) and ground, a pair of AC monitor circuits (24a, 24b) which are coupled to subscriber input terminals (3, 4) through the resistance network (19) and respective capacitors (25a, 25b), providing first outputs to the pair of current amplifiers (17a, 17b) through a second pair of nodes (c, c') to provide a predetermined terminal impedance to a speech signal across the subscriber input terminals (3, 4) and providing second outputs related to the speech signal across subscriber input terminals (3, 4) to the modem input terminals (T 0 , T 1 ) of a modem (28) through a third pair of nodes (e, e'), a pair of transmitters (26, 27) which receive a speech signal from the modem (28) through modem output terminals (R 0 , R 1 ), said transmitters (26, 27) adjusting said current amplifiers (17a, 17b) by controlling a fourth pair of nodes (d, d') in the control path of said current amplifiers by applying the output signal of the transmitters (26, 27) to said fourth pair of nodes (d, d'), a return signal detection circuit (32) receiving an output signal of one (26) of said transmitters (26, 27) through an impedance r x (31) which is coupled in parallel with another impedance r b (29) which couples between said one (26) of said transmitters and a further power source 30, said impedance r x (31) being equal to a terminal impedance of the present apparatus across the subscriber input terminals (3, 4), and said impedance r b (29) being equal to a line impedance of a subscriber line (2) across the subscriber input terminals (3, 4), the output of said return signal detection circuit (32) being applied to the third pair of nodes (e, e') where return signal is cancelled by said output of the return signal detection circuit (32), said return signal being originated across the subscriber input terminals (3, 4) and applied to said third pair of nodes (e, e') through the resistance network (19), the respective capacitors (25a, 25b) and the AC monitor circuits (24a, 24b).