This work presents a low power analog class-AB line driver with adaptive output impedance for Fast Ethernet. In order to provide a good matching to the line, a digital impedance control is implemented to adapt the output impedance of the line driver to the load connected to it. The advantage of this architecture is a low supply voltage requirement compared to voltage mode line drivers, while still maintaining a very low power consumption. A prototype of the line driver is fabricated in an 180 nm technology. Measurements demonstrate the ability to adapt to an output impedance in the range of 80 Ω to 130 Ω, while the 3dB bandwidth is 200 MHz. The power consumption is as low as 24.3 mW for the Fast Ethernet mode operating from 2.5 V. If the supply voltage is increased to 3.3 V the line driver is compatible to the older 10 Mb/s Ethernet standard requiring larger output voltages up to 5 Vpp. To achieve the low power consumption, an aggressive duty cycling approach is used. The impedance control is only activated a fraction of the time, to effectively lower the mean power consumption.