A circuit for generating a direct current voltage includes a three-axis antenna set, a first capacitor, a limiting direct current voltage generator, a second capacitor, and a low dropout regulator. The three-axis antenna set receives a signal transmitted by a reader. The first capacitor generates a first direct current voltage according to an X-axis component, a Y-axis component, and a Z-axis component of the signal. The limiting direct current voltage generator limits and converts the X-axis component, the Y-axis component, and the Z-axis component to generate an X-axis direct current voltage, a Y-axis direct current voltage, and a Z-axis direct current voltage. The second capacitor generates a second direct current voltage according to the X-axis direct current voltage, the Y-axis direct current voltage, and the Z-axis direct current voltage. The low dropout regulator generates a direct current output voltage according to the second direct current voltage.