Connection of telephone instruments for conference link - uses send and receive amplifiers with buffering to prevent feedback between microphone and earpiece

摘要

The circuit is for use in private branch exchanges or private telephone installations. The instruments on which the conference facility is available are connected together by special interconnectors. Each instrument is connected through a send and receive amplifier. Each receiver amplifier is connected through an ohmic resistor to one input of the transmit amplifier. When an instrument is not in use its transmit amplifier is loaded with a ballast resistor. The transmit amplifiers are connected to the interconnectors through damping resistors such that the receive and transmit circuits of an extension are not connected together. One of the instruments can be, and pref. is, available for connection to the public system. The private instruments are powered by constant current sources. The resistor values and connections are chosen to give the required damping between internal instruments and between them and the line connected instrument.