Telephone set, several electronic functions of which are powered by the line current and especially the function for amplified listening through loudspeaker

摘要

Telephone set, most of the functions of which are provided by electronic circuits supplied from the line current, and especially a transmission circuit 16 connected between the end portions 11 and 12 of the line wires with interposition of a resistor RM1 for measuring the current consumed by this circuit. Other function modules 40, 50 are powered from the end portion 12 via a circuit 27 for recovering line current and are connected moreover to the reference end portion 11 via another current-measuring resistor RM2. The voltage drops in the current-measuring resistors RM1, RM2 are kept equal by the action of a high-gain amplifier 30 which controls a follower transistor TS passing some of the line current required to keep this equality of voltage drops. According to an advantageous mode of implementation applied to amplified listening through loudspeaker 43, a current-mirror device 60 controls a reduction in gain of a preamplifier 50 for the amplified listening signal, when the line current is insufficient. Application to telephony. IMAGE