Design Of Tunable Gm-C Low-pass Filter Based On A High Linearity OTA

A filter is a system that eliminates noise and extracts characteristic information.It makes a kind of frequency signal component attenuates greatly and another kind of frequency signal component pass through,which realize the function of filtering out the noise,extracting the target information.In a wireless communication system,it is necessary to deal with a signal with a large amount of noise and to suppress the interference of the adjacent channel,so that the filter becomes a key block in the wireless communication system.At present,wireless communications industry professional network used a wide range of frequency and bandwidth.The frequency is mainly concentrated in the 100 MHz ~ 1.2GHz and the channel bandwidth is concentrated in the 5kHz-2MHz.Thus,a Gm-C low-pass filter,having a good tuning capability from 5kHz to 2MHz,is designed in this paper to meet the different needs of private networks.As an important part of the Gm-C filter,its linearity severely limits the overall dynamic range of the filter circuit.Thus,an improved linearly OTA which can tuned linearity is presented in this paper.The proposed OTA employs two linearization techniques of input attenuators and cross-coupled double differential pairs.The OTA achieves a 20 times relative tuning range without decreasing the input voltage-swing,which is enabled by using the source-degenerated current mirrors.A six-bit DAC is designed to provide bias current for Gm tuning,for which realize the digital tuning capability for cut-off frequency of this filter.A third order low-pass filter using the proposed OTA is designed by cascading biquad structure.In order to obtain 400 times relative adjustment range,the cut-off frequency is typically adjusted by implementing Gm-tuning and capacitor-switching.Gm-tuning achieves the fine tuning and Capacitor-switching achieves the coarse tuning.At the same time,the common-mode feedback circuit is shared.Benefited from this method,the power consumption and area can be reduced efficiently.The proposed Gm-C low-pass filter is designed with the 0.18 m SIMC CMOS process.The power consumption of this filter is 0.9–1mW with 3.3V supply voltage.The post-layout simulation results show that the filter exhibits a linear tuning capability from 5kHz to 2MHz and the in-band input 3rd order intercept point(IIP3)remains more than 14.2dBm.The variation range of simulated spectral density of input referred voltage noise is 257nV/(?) to 27 mV/(?).