This work presents and analyzes a single-ended to differential converter (balun) used as a building block in multiplexers for extending laboratory measurement capabilities at high data rates. It is based on a differential amplifier topology that is stimulated single-endedly. To prove the concept, the design is realized in a 130 nm SiGe BiCMOS process. The circuit has an active area of 0.03 mm ² and a power consumption of 164 mW. It achieves a gain of 13 dB,< | 8|° phase and < | 1.2 mid dB gain imbalance over the whole measured bandwidth of 63 GHz and the highest reported measured data rate of 120 Gb/s. To the best of our knowledge, this is the first measured active balun that demonstrates such high data rates.