A general purpose impedance tuner is proposed for adaptive matching applications to compensate the impedance variations of devices such as antennas or power amplifiers. The impedance tuner, composed of a phase shifter and a variable transformer, is designed based on all-pass networks. Thin-film barium-strontium titanate (BST) parallel-plate capacitors fabricated on sapphire are used as the tuning elements. The impedance tuner is completed by mounting chip inductors and blocking capacitors on the same substrate. On-wafer measurement results show that, at 1.8 GHz, impedance transformation ratio of 4 can be achieved and the dissipation loss for all biasing voltages (0 to 18 V) is less than 5.5 dB.