An enhanced adaptive Q-tuning scheme for a 100-MHz fully symmetric OTA-based bandpass filter

A tuning scheme for continuous-time high-Q biquad filters is presented. An improvement over the existing implementation of the modified-LMS Q-tuning scheme is proposed and efficiently combined with the frequency tuning based on phase-locked loops. The proposed scheme takes much less area without compromising the accuracy achieved previously. The proposed unified Q- and f/sub 0/-tuning scheme does not require the Q-tuning loop to be slower than the f/sub 0/-tuning loop. The optimal case is to have equal speeds for both loops. Also, a low-voltage pseudo-differential operational transconductance amplifier with inherent common-mode feedforward is introduced. The structure is fully symmetric and suitable for high-frequency applications. An experimental test chip is fabricated in standard CMOS 0.5-μm technology, with a bandpass filter of center frequency 100 MHz and Q of 20, along with the proposed tuning scheme. The measured Q-tuning error is around 1%. Expected and experimental results are in good agreement.