An on-chip implementation of a low loss 1:9 transmission line transformer (TLT) is presented. The TLT was fabricated with broadside coupled microstrip lines in a GaAs HBT process. The wide metal transmission lines allow the high-quality factor and the high-current capability. The measured insertion loss is 0.88 dB at 880 MHz. The equivalent circuit model of the TLT is also presented. The model shows good agreement with the measured data. (C) 2006 Wiley Periodicals, Inc.