Wide Range Tunable Transconductance Filters

Abstract

Generation of filters using operational transconductance amplifiers and capacitors (OTA-C) has become more and more popular over recent years. This is due to the need for high quality analog filters that can be implemented on a standard CMOS integrated circuit. While operational amplifier based active filters have served this purpose in the past, the OTA-C filter has advantages over a classic amplifier design. A simple yet effective method of analytical synthesis was developed that makes use of ladder circuits, which are widely understood. Given the ladder circuit of a filter of any order, an equivalent OTA-C filter can be easily synthesized. The synthesis process is shown for both a low pass prototype filter, and its band pass transformation. Higher order filters can easily be synthesized by extending the design process described. An OTA was developed which was designed specifically to address the complications involved in implementing filters with high quality factors. The design of a band pass filter is then implemented in CMOS with the OTA to show the effect of non-ideal elements. The CMOS filter implementation is capable of achieving a quality factor of 20 up to frequencies of 100 kHz. The reference current can be used to tune the filter center frequency linearly over three decades. This flexibility in center frequency tuning can be used to compensate for process variations or for creating general purpose filters with constant passive element values.