Automatic gain controller

An automatic gain controller, suitable for an integration circuit of an audio amplifier, the integration circuit comprising an operational amplifier, the automatic gain controller comprising:

• a plurality of resistors, connected in series between an input terminal and an inverting input terminal of the operational amplifier, the input terminal for receiving an input signal;
• a plurality of switches, each of which having a first terminal coupled to a corresponding one of the plurality of resistors, and a second terminal coupled to the inverting input terminal of the operational amplifier, the plurality of switches respectively controlled by a plurality of switching signals so as to be turned on or turned off;
• a buffer, coupled to the inverting input terminal of the operational amplifier, and generating a buffer signal according to a voltage at the inverting input terminal, the voltage at the inverting input terminal being associated with the input signal;
• a charge pump circuit, coupled to the buffer to receive the buffer signal, and generating a boost voltage according to the buffer signal and a clock signal;
• and
• a voltage level shifter, coupled to the plurality of switches and coupled to the buffer and the charge pump circuit to respectively receive the buffer signal and the boost voltage, and converting voltage levels of the control signals according to the buffer signal and the boost voltage to generate and respectively output the switching signals to the plurality of switches.