Miniaturized BAW Filter for Wide Band Application Based on High-Q Factor Active Inductor

Area of Science:

• Electrical Engineering
• Radio Frequency (RF) Engineering
• Materials Science

Background:

• Bulk Acoustic Wave (BAW) filters are crucial in RF circuits.
• Integrated passive inductors in BAW filters are large and have low quality factors (Q), degrading performance and increasing size.
• Miniaturization and performance enhancement of BAW filters are essential for modern packaging requirements.

Purpose of the Study:

• To present a miniaturized wide-band BAW filter utilizing a small-size, high-Q active inductor.
• To overcome the limitations of traditional passive inductors in BAW filter applications.
• To demonstrate significant improvements in BAW filter size and performance.

Main Methods:

• An active inductor was designed using a three-stage common-source amplifier topology with N-type transistors.
• A small transistor in the middle stage was employed to minimize parasitic capacitance and broaden inductive bandwidth.
• The active inductor was integrated into a 4.55-5.05 GHz BAW filter ladder structure.

Main Results:

• The active inductor demonstrated variable inductance (1-10 nH) and a high Q-factor (up to 4000) across a wide frequency range (2-7 GHz).
• The miniaturized active inductor occupied a chip area of 30 × 30 μm².
• The resulting BAW filter exhibited low insertion loss (-1.1 dB) and high out-of-band rejection (-35 dB left, -53 dB right).

Conclusions:

• The proposed active inductor significantly reduces the overall chip size of BAW filters to 0.83 × 0.75 mm².
• This active inductor approach substantially improves BAW filter performance compared to designs using passive inductors.
• The developed BAW filter is suitable for applications demanding compact size and high RF performance.