A power supply assembly that can be miniaturized even though an applied voltage to a load is rendered variable, and a semiconductor testing system using the same are put into practice. With an improvement of the power supply assembly for finding an error against a set voltage by feeding back an applied voltage applied to a load, and applying a predetermined voltage to the load by causing an output amplifier to increase and decrease amperage to be fed to the load on the basis of the error, it is characterized in provided a voltage converter causing a voltage level of a power supply voltage of the output amplifier to follow up a voltage level of the set voltage.