A low voltage linearly tuned fully differential CMOS OTA and its applications in filter design

This paper presents an adaptation of a transconductor built around MOSFETs operating in saturation. The circuit operates off a 3.3 V single supply and has a fully differential output. A common mode feedback circuit maintains the output near midsupply. Simulation results show that the transconductor is linearly tunable over a wide range of bias voltages. The transconductor is used to realize a second order filter in TSMC (Taiwan Semiconductor Manufacturing Corporation) 0.18 μm CMOS technology. Tuned by a DC voltage V_fo the filter center frequency was measured to be almost linearly adjustable from 833 kHz up to 1.3 MHz. The common mode voltage at the filter output was also observed to vary by no more than 24 mV about the desired value. A maximum THD of -35 dB is guaranteed at V_out=600 mV_pp within the tuning range of the filter.