Fujitsu Corporation

Yozo Iketani

Kenji Takato

Toshiro Tojo

Yuzo Yamamoto

Takashi Sato

Seiji Miyoshi

Kazumi Kinoshita

Shin-Ichi Itoh

Mitsutoshi Ayano

Kiyoshi Shibuya

Hiroyuki Ujiie

Shin-ichi Ito

An ISDN interface circuit is provided which is coupled to a terminal device via a transmission line and a reception line, the transmission line being formed of a pair of first and second wires, and the reception line being formed of a pair of third and fourth wires. The ISDN interface circuit includes a first resistor and a second resistor connected in series at a first node and between the first and second wires of the transmission line, and a third resistor and a fourth resistor connected in series at a second node and between the third and fourth wires of the reception line. The ISDN interface circuit also includes a D.C. power source connected to one of the first node, at which the first resistor and the second resistor are mutually connected in series and the second node, at which the third resistor and the fourth resistor are mutually connected in series. One of the first node and the second node, other than the above-mentioned one of the first node and the second node connected to the D.C. power source, is grounded.

ISDN interface circuit and system using the same

Ring trip circuit for subscriber telephone

Disclosed is a terminating circuit in a battery feed circuit of an electronic exchange system. The battery feed circuit comprises two transconductance amplifiers connected by a coupling capacitor, for supplying a direct current through a ring terminal and a tip terminal to a load. Each of the transconductance amplifiers comprises an operational amplifier; a resistor connected between the output end of the operational amplifier and the tip terminal or the ring terminal, and an impedance unit connected between the inverting input terminal of the operational amplifier and the output end. The impedance unit has an impedance determined in such a way that the required terminating impedance of the battery feed circuit when viewed from the tip terminal and the ring terminal is formed only by the internal elements including the impedance means and the resistor, whereby an external larger-size capacitance capacitor for matching becomes unnecessary.

Terminating circuit in a battery feed circuit of an electronic exchange

A subscriber line interface circuit receives and twice inverts currents from a 2-wire subscriber line and returns the currents to the subscriber line so longitudinal input impedance is low. Any differential input current in the subscriber lines is bypassed by a capacitor, so differential input impedance is very high. The circuit also sends a signal current from one system having one ground to another system having a different ground through one current path, while sending another current, having a phase opposite and amplitude equal to the signal current, through another path. This effectively isolates the two systems having different grounds.

Subscriber line interface circuit

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Yokohama, Japan

Yozo Iketani