A 10 Gbit/s modulator driver in SiGe 0.25 m BiCMOS technology, which is supplied by 2.5 and 3.5 V with a low power consumption of 680 mW and a differential output voltage swing of 5 Vpp is presented. The non-inverted as well as the inverted outputs deliver a voltage between 0 and 2.5 V so that no bias-tee is needed at the output. Additional to the low-power design, a passive network instead of an additional amplifier circuit for driving the cascode transistors, which limit the collector-emitter voltage of each transistor in the output stage below breakdown, is presented. According to bit error rate (BER) measurements with a pseudorandom bit sequence (PRBS) with the length 231 - 1 the BER is better than 10-12 to input voltage differences down to 40 mVpp. The rise/fall (20 to 80%) time is 45ps/30ps, respectively. The modulator driver has been developed for integration together with a silicon optical phase modulator.