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