A 1.9 GHz Si active LC filter with on-chip automatic tuning

The achievable quality factors (Q) of on-chip inductors fabricated with standard Si technology are limited. Inductor Q factors can be enhanced by active means, resulting in GHz-range filters with adequate dynamic range for certain applications. One such application is the replacement of off-chip filters in the transmit part of a wireless transceiver, following a mixer. The limits of the technique are tested here by using it in a filter for the entire band used in the PCS 1.9 GHz standard. A tuning-friendly filter can be built with magnetically coupled LC resonators. Two identical Q-enhanced LC resonators, together with the loading impedances, are used form a balanced 4th-order bandpass filter. The filter and its tuning system are implemented in a 0.25 μm BiCMOS technology