A tunable DC voltage generating circuit includes: a resonance circuit including an inductor and an input capacitor connected in series configuration, and arranged to operably receive an input signal and generating a resonance signal at an output node between the inductor and the input capacitor; a rectifying circuit coupled with the output node and arranged to operably rectify the resonance signal; a current control unit connected with the resonance circuit in series configuration; a stabilizing capacitor coupled with an output terminal of the rectifying circuit and arranged to operably provide a DC output signal having a voltage level greater than that of the input signal; and a control circuit coupled with the output terminal of the rectifying circuit and the current control unit, and arranged to operably adjust a current passing through the current control unit to thereby change the DC output signal.