Design And Research Of Trans-conductance Capacitor Low-pass Filter

Filters play an important role in signal processing.A low-pass filter is an electronic device that can pass signals below a certain frequency and attenuate signals above the frequency.There are various kinds of filters,and trans-conductance capacitor filter is one of them.It consists of operational trans-conductance amplifiers and capacitors,which is also called G_m-C filter.Able to work in open-loop,G_m-C filter is not very sensitive to the bandwidth and can work in a wide range from the intermediate frequency to the high frequency.Consequently,it is widely used in the radio transceiver system and has become a research hotspot in recent years.Based on the prototype of sixth-order Chebyshev I filter,a G_m-C filter low-pass filter is designed in this dissertation,of which the pass-band is wider than 100 MHz,the in-band ripple is less than 1d B and the stop-band attenuation at 200 MHz is more than 40dB.This dissertation introduces the basis of the design of filters first,and then summarizes a set of standardized design process of low-pass filters.According to the design specifications,the normalized transfer function of the sixth-order G_m-C low-pass filter designed in this dissertation is derived.After that,the filter is modeled by three cascaded general biquad circuits.Aimed at optimizing the linearity,noise and power consumption of the filter,an efficient strategy is applied in the design of biquadratic sections.The following step analyses the linearization technique of the operational trans-conductance amplifier,which is the core module in the G_m-C filter.Five different structures are compared and finally a trans-conductance amplifier with dynamic source attenuation is chosen.With this structure,the minimum G_m cell has a linear range of ± 350 m V and the maximum G_m cell has a linear range of ± 200 m V.At last,this dissertation presents a complete filter system in Cadence,consisting of a switch control unit,a constant-transconductance bias circuit,an adjustable current bias,three cascaded filters and a buffer.Besides using the constant-transconductance self-bias to provide the main current bias,there is also a spare constant-current source controlled by the off-chip voltage in the system,to some extent making the system tunable and reducing the mismatch of G_m changing with external environmental factors.The G_m-C filter is fabricated in TSMC 90 nm CMOS process,and the power supply is 1.2V.The whole chip occupies an area of 385um×6 47 um,the core area of which is 385um×398um.Simulation results show that under the condition of the constant-transconductance self-bias coupled with the tuning of the adjustable current bias,the in-band ripple,cut-off frequency and stop-band attenuation can meet requirements with the temperature changing from 0 ? to 70?.At a typical temperature of 50?,the overall current consumption is 3.2m A,the in-band ripple is 0.75dB,the-3d B bandwidth is 109 MHz,the attenuation is 49dB at 200 MHz and the third-order intercept point(IIP3)is 8.7d Bm.