Fujitsu Corporation

Kazumi Kinoshita

Kenji Takato

Toshiro Tojo

Yuzo Yamamoto

Shin-Ichi Itoh

Kazuyuki Minohara

Kiyoshi Kurosaki

Yozo Iketani

Shin-ichi Ito

Taichi Kosako

A ring trip detection circuit detects an OFF-hooked state of a telephone receiver during ringing of a telephone set by detecting a change in a voltage across two terminals of a transmission resistor through which a ringing signal is supplied to the telephone set from a ringing signal source. The ring trip detection circuit includes a voltage dividing circuit coupled to the two terminals of the transmission resistor for dividing voltages at the two terminals and for outputting divided voltages, a pair of transistors having bases for respectively receiving the divided voltages from the voltage dividing circuit, a constant current source, a pair of resistors having first terminals respectively coupled to emitters of the pair of transistors and second terminals coupled in common to the constant current source, where the pair of resistors have the same resistance, and a current mirror circuit coupled to collectors of the pair of transistors for outputting an output current which indicates whether or not the telephone receiver is OFF-hooked.

Ring trip detection circuit

Ground fault detector

Ring trip circuit for subscriber telephone

A battery feed circuit including a main battery feed part and its peripheral circuits provided in a line circuit of a switching system. The peripheral circuits include the following: First, a means for forming a low impedance with respect to in-phase signals, simply realized by a capacitor; second, a bias circuit able to suppress the entry of power source noise and easily switch magnitudes of bias voltages and polarities, realized by a bias voltage generating means, a buffer means, and and AC bypass capacitor; third, a battery feed current control circuit able to suppress overflow of battery feed current when the distance to the subscriber is short, which detects an overcurrent component above a predetermined value, and, in accordance with the same, changes the bias voltage; fourth, a battery feed current supervising circuit able to detect current in both the forward and reverse directions, realized by an emitter-follower pair and a voltage/current converting resistor; fifth, a circuit for detecting bidirectional battery feed currents at a high precision, which is formed by forward direction and reverse direction current detection means provided with V.sub.BE cancelling means; and sixth, an overcurrent protection circuit, which changes the voltage applied between the inputs (+,-) of a transconductance amplifier forming the main battery feed part in accordance with an excess voltage.

Battery feed circuit

An operational amplifier for producing an output signal representing the voltage differential between two input signals is powered by first, second and third power sources respectively having high, intermediate and low output voltage levels. A voltage buffer circuit, which is connected to the second power source, interconnects a constant current source, which is connected to the first power source, and an amplifier means, which is connected to the third power source, and separates the constant current source from the third power source. A differential amplifier in the amplifier means responds to the two input signals for producing a voltage differential output signal, and an output circuit connected between the second and third power sources receives the voltage differential output signal of the differential amplifier for producing the voltage differential output signal of the operational amplifier.

Operational amplifier

A protection circuit for a battery feed circuit used in a switching system. The protection circuit is comprised of: a supervising circuit for detecting an abnormal current flowing through a battery feed resistor; and a voltage limiting circuit for clamping a voltage across the battery feed resistor when the abnormal current is detected. Further, a control circuit is employed for stopping an operational amplifier, which is a component of a conventional battery feed circuit, when the abnormal current is detected.

Protection circuit for battery feed circuit

A constant current circuit, including a constant current source which draws a current and a constant current source which sends a current, is connected to a two-wire subscriber line. A ringing signal made up of constant stream of positive and negative current signals is transmitted to the telephone line. The OFF-hook condition of a telephone set is detected by comparing the output voltage of a constant current circuit with a reference voltage for ring trip.

Growing community of inventors

Kawasaki, Japan

Kazumi Kinoshita