PURPOSE:To improve an output voltage control characteristic, by a method wherein a second PNP transistor is inserted into a shunt circuit to amplify an output voltage while a variable impedance element is inserted therein. CONSTITUTION:A DC-DC converter is constituted of a DC power source 1, a transformer 2, a transistor 4, a diode 13, provided in the secondary winding 12 of output side, and the like to supply electric power to a load 15. Said transistor 4 is driven by the feedback coil 6 of the transformer 2 through a resistor 7 and a capacitor 8. In this case, a variable impedance element 16 is inserted between the emitter and the base of a second transistor 17, inserted into a shunt circuit, to control the Zener current of a Zener diode 11 for generating a reference voltage which determines an output voltage. According to this method, a Zener voltage may be kept constant in appearance and the variability of the products of Zener operating voltage may be eliminated.
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