Telephone answering machine interface circuit
AssigneeT.A.D. Avanti, Inc.
InventorStephen V. Lyle
Life of this patent
Estimating the term…
Abstract
An interface circuit is provided for a telephone answering machine which does not require a transformer and which provides the proper direct current and alternating current terminating impedance for the telephone line. The direct current terminating impedance is provided by connecting a resistor of a selected resistance value across the telephone line, and the alternating current terminating impedance is achieved by multiplying the effective impedance of the resistor for alternating current audio signals by providing an amplifier for the outgoing audio signals with positive feedback of amounts sufficient to provide the desired impedance multiplication effect but which is insufficient to sustain oscillation.
Claims5
A circuit for interfacing a telephone answering machine to a telephone line, including:
- a transmit amplifier circuit for audio signals to be introduced to the telephone line and including first and second differential amplifiers;
- circuitry for connecting the output of said amplifier circuit to the telephone line including an output circuit connected to said differential amplifiers to provide a balanced output between the outputs of said differential amplifiers;
- a resistor of a selected resistance value included in said circuitry adapted to be connected across said telephone line to provide a direct current terminating impedance for the telephone line;
- and a positive feedback circuit connected to said transmit amplifier circuit to enable said transmit amplifier circuit to increase the effective impedance of said resistor to an increased apparent alternating current impedance in the presence of audio signals.
Dependencies are inferred from the claim text.