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

## TL;DR

This paper introduces a miniaturized BAW filter using a high-quality active inductor to improve performance and reduce size for RF applications.

## Contribution

A novel three-stage active inductor design with high Q factor and small size for BAW filters is proposed.

## Key findings

- The active inductor achieves a quality factor of up to 4,000 from 2 to 7 GHz.
- The BAW filter with the active inductor has an insertion loss of −1.1 dB and strong out-of-band rejection.
- The filter occupies a chip size of 0.83 × 0.75 mm², significantly smaller than traditional designs.

## Abstract

BAW filters have been widely used in RF circuits, and their combination with integrated passive inductors is one of the most common forms of BAW filters. However, the large size of passive inductors increases the area of the filter, making it unable to meet packaging requirements. At the same time, their low quality factor (Q) severely degrades the performance of the BAW filter. This paper presents a miniaturized wide band BAW filter with small-size high-Q active inductor. The active inductor is implemented by a circuit topology with three common-source amplifiers constructed with N-type transistors. The three-stage topology uses a small-size transistor in the middle stage to reduce the parasitic capacitance at the input node, achieving a large inductive bandwidth. The simulation results show that the active inductor has variable inductance from 1 nH to 10 nH, and a quality factor of up to 4 K from 2 to 7 GHz. The 30 × 30 μm2 active inductor is embedded in a 4.55–5.05 GHz BAW filter ladder so as to substantially decrease filter size. Simulation results indicate that the BAW filter based on the active inductor achieves a low insertion loss of −1.1 dB, out-of-band rejection of −35 dB on the left side, and out-of-band rejection of −53 dB on the right side. Compared to the traditional passive inductor, this active inductor significantly improves the performance of the BAW filter while occupying a much smaller chip size of 0.83 × 0.75 mm2.