Patent ID: 7227409

Claim:
An apparatus for removing DC offset and amplifying signal with variable gain simultaneously, comprising: a voltage following circuit for providing a reference voltage level; a first amplifier module, comprising: a first resistance, comprising one terminal coupled to a positive input signal; a first operational amplifier, comprising an inverting terminal coupled to another terminal of the first resistance; and a second resistance, comprising one terminal coupled to the inverting terminal of the first operational amplifier, and another terminal coupled to an output terminal of the first operational amplifier; a first filter circuit, comprising: a third resistance, comprising one terminal coupled to the output terminal of the first operational amplifier; a first capacitor, comprising one terminal coupled to one terminal of a fourth resistance, and another terminal connected to ground; a second amplifier module, comprising: a fourth resistance, comprising one terminal coupled to the positive input signal; a fifth resistance, comprising one terminal coupled to an output terminal of the first filter circuit; a second operational amplifier, comprising a non-inverting input terminal coupled to the voltage following circuit and an inverting input terminal coupled to another terminals of the fourth resistance and the fifth resistance; and a sixth resistance, comprising one terminal coupled to the inverting terminal of the second operational amplifier, and another terminal coupled to an output terminal of the second operational amplifier; wherein the fourth resistance is R 4 , the first resistance is R 1 , the second resistance is R 2 and the fifth resistance is R 5 , and wherein R 4 = R 5 × R 1 R 2 .