a power source apparatus is disclosed in this invention , specifically a power source apparatus comprising a voltage amplitude control unit that employs an active power factor corrector to control the output dc voltage by applying positive or negative logic control voltage , incorporated with high frequency power source circuit and high frequency transformer , brightness of cold cathode fluorescent lamp or external electrode fluorescent lamp are controllable and dc power is directly applied to dc load . such method is accomplished by adjusting the amplitude of the supplying dc voltage for controlling the amplitude of the high frequency voltage of ccfl or eefl , thus called voltage amplitude method . because of the characteristics of stable frequency , high resolution and linearity , vam is broadly used in the control of luminance of discharge tubes , such as tft - lcd tvs , lcd monitors and advertisement lamps . the impulse width controller of the present invention achieves the luminance control of ccfl or eefl inside or outside the glow discharge zone .