Patent ID: 7741905

Claim:
An automatic gain control circuit for controlling gains of audio signals outputted from an audio device, the automatic gain control circuit comprising: a first voltage dividing resistor that divides a voltage of the automatic gain control circuit using an internal impedance of the audio device to produce a divided DC voltage, wherein the first voltage dividing resistor is connected between a first power source and an output of the audio device; a filter circuit that filters audio signals outputted from the audio device to produce filtered audio signals, and wherein the filter circuit receives the divided DC voltage, wherein the filter circuit is connected to the output of the audio device, wherein the filter circuit comprises a first current limit resistor with one end connected to the output of the audio device and the other end being the output end of the filter circuit, and a filtering capacitor with one end connected to the first current limit resistor and the other end grounded; a switch circuit that outputs controlling signals according to the divided DC voltage from the filter circuit; and an amplifying circuit that amplifies the filtered audio signals according to the controlling signals, wherein the amplifying circuit comprises a first input end that receives the filtered audio signals, a second input end that receives the controlling signals from the switch circuit, and an output end that outputs the amplified audio signals, wherein the first input end of the amplifying circuit is connected to the audio device via a first capacitor.