Abstract: A power source circuit having a reference voltage generating circuit for generating a DC reference voltage a current output circuit provides across output lines a current proportional to an input voltage, and an error voltage integration circuit is provided for integrating a voltage proportional to a difference between the DC reference voltage and the voltage across the output lines at a required integration time constant to apply the integrated output as the input voltage to a current output circuit. The integration time constant is set so that an output terminal connected to the output lines may have a low output impedance in terms of direct current to maintain a predetermined constant DC output voltage and have a high output impedance with respect to an AC signal applied to the output lines.