Reconfigurable RF active bandpass filter with BaxSr1-xTiO3 varactor

An RF tunable active bandpass filter employing a Ba_xSr_1-xTiO₃ (BST) based capacitor is proposed. This filter consists of an active capacitance circuit together with a BST tunable varactor. The active capacitor is made of a field effect transistor that exhibits negative resistance as well as capacitance. BST thin film was deposited by Sol-Gel solution and using the metal-insulator-metal capacitor process. The fabricated capacitor showed capacitance of 2.7-1.5 pF at 1.8 GHz for a maximum applied electric field of 34 V/ mum. The measured second- order active bandpass filter showed bandwidth of 100 MHz, insertion loss of+0.48 dB at 1.8 GHz centre frequency and tuning frequency of 220 MHz with the application of 34 V/ mum.