Abstract: A power circuit is comprised of a control transistor connected in series between one terminal of a power source and one terminal of a load, an error detecting amplifier circuit for detecting and amplifying changes of the load voltage, a drive transistor for driving the control transistor in response to the output current from the error detecting amplifier circuit, an input-output voltage difference detecting transistor which conducts when the difference between the input and the output voltages of the control transistor decreases to below a predetermined value, and a shunt transistor for shunting the current supplied to the drive transistor when it is activated by the output current of the input-output voltage difference detecting transistor. The power circuit operates as a constant voltage circuit when the input voltage is not lower than the predetermined value, while it operates as a ripple filter with a wide operable range when the input voltage decreases to below the predetermined value.