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This paper reports for the first time on a second order lattice balun (SOLB) implemented in commercial GaAs MMIC technology. Over a 90 % bandwidth 1.5-3.5 GHz, the measured amplitude and phase errors are less than 1.3 dB and 4°, respectively. Over the same frequency range the reflection loss is better than 20 dB, and the insertion loss less than 1.0 dB with a minimum of less than 0.6 dB at the center frequency 2.5 GHz. The effective area of the fabricated circuit is 620 × 650 μm. For the sake of comparison a conventional direct-coupled transformer balun is designed in the same technology. The transformer balun is smaller, 370 × 420 μm2 and it exhibits good balancing performance. However, its insertion loss is worse than 3 dB when the circuit is matched.