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This letter proposes a transmission-line transformer (TLT) with high impedance-transformation ratio of 1 : 9 for wideband power amplifier design. The 1 : 9 TLT is realized with broadside-coupled and multiple-metal stacked transmission lines and achieves a broadband impedance transformation from 5.0 ±0.1Ω optimal load impedance of the power cell to 50-Ω load with a bandwidth of 4.4 to 6.6 GHz, which covers the required bandwidth of the IEEE 802.11a WLAN application. The measured minimum insertion loss is 1.07 dB at 5.8 GHz with a 3-dB bandwidth of 164%. This 1 : 9 TLT is fabricated in standard 0.18-μm CMOS process with a chip area of 426 μm × 589 μm including the test pad.