the invention features an output driver for integrated circuits that includes a driver that has a data input connected to the integrated circuit , a data output connected to a transmission line leading to the external circuit , and impedance adjusting means for adjusting the output impedance of the driver circuit according to determinable impedance adjusting data . the output driver also includes a dummy circuit having a dummy driver circuit and transmission line , and an impedance control circuit for controlling the output impedance of the driver circuit . the impedance control circuit controls the impedance of the driver circuit by determining the impedance adjusting data ( necessary for matching the output impedance of the dummy driver circuit to the characteristic impedance of the dummy transmission line and outputting the determined impedance adjusting data to the impedance adjusting means of the driver circuit .