A 3rd Order Butterworth Gm-C Filter for WiMAX Receivers in a 90nm CMOS Process

This paper presents a design of a 3 rd order Butterworth Gm-C Low Pass Filter (LPF) for WiMAX receivers in a 90 nm CMOS process. Due to its robustness to process parameter tolerances, a passive LC-ladder filter was emulated using the Signal Flow Graph (SFG) method. As a building block for the LPF, a highly linear operational transconductance amplifier (OTA) based on bias-offset cross-coupled differential pairs was designed and characterized. With this OTA's property of having its transconductance linearly controlled by a DC voltage bias, the corner frequency of the filter can be externally tuned to achieve the WiMAX specifications for bandwidth. From layout simulations, the LPF achieved a bandwidth of 10 MHz at 0.5V DC bias, under a 1V supply. HD2 at 50 mV peak, 1KHz input is-40.38 dBc. With the addition of a source follower at the output, the filter attained good output matching, with an S22 of -16.45 dB. The total power dissipation is 5.26 mW.