| 摘要 |
To allow a universal replacement telephone handset, an adaptive amplifier circuit uses DC power available from a host telephone set base to discriminate between low resistance and high resistance base power supplies. It emulates either a carbon microphone or an electret microphone, whichever is appropriate for the set base. The circuit couples a microphone (10) and an output terminal (2a) through a series connection of resistor (R1), input and output leads of an amplifier (12), and a sense circuit (22). The sense circuit controls two current sources (14),(16) in response to the shunt voltage and series current available from the telephone base. Current source (14) connects the amplifier output to an input terminal (1a) and acts either as a bias current source, or as an electronic switch that provides a short circuit from the microphone to the the amplifier output. Current source (16) connects in series between the amplifier ground lead and a common terminal (2b) and acts either as a bias current source, or as an electronic switch to enable or disable the amplifier. The sense circuit has negligible series or shunt effect on voice signals except when a carbon microphone is being emulated, at which time it provides additional amplification to the voice signals at the amplifier output. Overall amplification is controlled by a negative feedback loop (20) which connects from terminal (2b) to the the amplifier input.
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