An amplifier circuit (100, 300, 400), comprising: an operational amplifier (110, 310, 410) having an input terminal and an output terminal, for receiving an input signal to generate an output signal; a feedback path coupled between the input terminal and the output terminal of the operational amplifier (110, 310, 410), wherein the feedback path comprises two poly resistors (R1, R2, R3), wherein each of the two poly resistors (R1, R2, R3) comprises a corresponding depletion region which indicates a corresponding conductive layer which is isolated from a corresponding resistive layer by a corresponding dielectric layer, the depletion region of one of the two poly resistors (R1, R2, R3) is biased by the output signal generated by the operational amplifier (110, 310, 410), the operational amplifier (110, 310, 410) is configured to generate the output signal to the corresponding conductive layer of the one of the two poly resistors (R1, R2, R3) to directly bias the corresponding depletion region the corresponding depletion region of the other one (R2, R3) of the two poly resistors (R1, R2, R3) is configured to be floating as being not connected to any reference voltage, and the two poly resistors (R1, R2, R3) are connected in series, wherein the one of the two poly resistors (R1, R2, R3) is coupled to the output terminal of the operational amplifier (110, 310, 410), and the other one (R2, R3) of the two poly resistors (R1, R2, R3) is coupled to the input terminal of the operational amplifier (110, 310, 410). an operational amplifier (110, 310, 410) having an input terminal and an output terminal, for receiving an input signal to generate an output signal; a feedback path coupled between the input terminal and the output terminal of the operational amplifier (110, 310, 410), wherein the feedback path comprises two poly resistors (R1, R2, R3), wherein each of the two poly resistors (R1, R2, R3) comprises a corresponding depletion region which indicates a corresponding conductive layer which is isolated from a corresponding resistive layer by a corresponding dielectric layer, the depletion region of one of the two poly resistors (R1, R2, R3) is biased by the output signal generated by the operational amplifier (110, 310, 410), the operational amplifier (110, 310, 410) is configured to generate the output signal to the corresponding conductive layer of the one of the two poly resistors (R1, R2, R3) to directly bias the corresponding depletion region the corresponding depletion region of the other one (R2, R3) of the two poly resistors (R1, R2, R3) is configured to be floating as being not connected to any reference voltage, and the two poly resistors (R1, R2, R3) are connected in series, wherein the one of the two poly resistors (R1, R2, R3) is coupled to the output terminal of the operational amplifier (110, 310, 410), and the other one (R2, R3) of the two poly resistors (R1, R2, R3) is coupled to the input terminal of the operational amplifier (110, 310, 410). each of the two poly resistors (R1, R2, R3) comprises a corresponding depletion region which indicates a corresponding conductive layer which is isolated from a corresponding resistive layer by a corresponding dielectric layer, the depletion region of one of the two poly resistors (R1, R2, R3) is biased by the output signal generated by the operational amplifier (110, 310, 410), the operational amplifier (110, 310, 410) is configured to generate the output signal to the corresponding conductive layer of the one of the two poly resistors (R1, R2, R3) to directly bias the corresponding depletion region the corresponding depletion region of the other one (R2, R3) of the two poly resistors (R1, R2, R3) is configured to be floating as being not connected to any reference voltage, and the two poly resistors (R1, R2, R3) are connected in series, wherein the one of the two poly resistors (R1, R2, R3) is coupled to the output terminal of the operational amplifier (110, 310, 410), and the other one (R2, R3) of the two poly resistors (R1, R2, R3) is coupled to the input terminal of the operational amplifier (110, 310, 410).