| 摘要 |
<p>An adaptive amplifier circuit for a replacement handset emulates either a carbon microphone or an electret microphone, whichever is appropriate for the telephone base. The circuit couples microphone (10) and 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 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 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. Overall amplification is controlled by negative feedback loop (20) which connects from terminal (2b) to the amplifier input.</p> |
