An operational circuit includes: a gain control circuit arranged to provide a gain value upon an input signal according to a set of control signals, wherein the gain control circuit includes a first resistor-based network and a second resistor-based network; an operational amplifier coupled to the gain control circuit and arranged to generate an output signal according to the input signal and the gain value; and a first capacitor coupled to the operational amplifier and arranged to hold the output signal between a first input terminal and a first output terminal of the operational amplifier, wherein when the operational circuit is operating, a first terminal of the first capacitor is consistently coupled to the first input terminal of the operational amplifier, and a second terminal of the first capacitor is consistently coupled to the first output terminal of the operational amplifier.