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Design of 9:4 Step-Down Impedance Transformation Ruthroff Balun with Center Tap in GIPD Technology

This paper presents a 9:4 step down impedance transformation in glass integrated passive device (GIPD) technology. Unlike traditional Ruthroff balanced-to-unbalanced (balun) having a 1:4 step-up impedance transformation ratio, the proposed 9:4 step-down balun is realized by cascading a 9:1 unto-un transmission line transformer (TLT) and a 1:4 TLT balun. Since the balanced port exhibits low impedance, the proposed step-down balun is suitable for the power combining and impedance transformation for high power amplifier design. Particularly, a center tap with bypass capacitor at the middle of balanced line is intentionally designed to use as DC biasing for the power transistors. The minimal insertion loss is 0.86 dB at 5.3 GHz and the 1-dB bandwidth is from 4.2 GHz to 7 GHz, which is correspondent to 51.6% of fractional bandwidth.