Design of highly linear tunable CMOS OTA for continuous-time filters

An analytical method based on the standard square-law metal-oxide-semiconductor (MOS) modeling for the design of highly linear, fully differential complementary metal-oxide-semiconductor (CMOS) operational transconductance amplifier (OTA) is presented. The proposed circuit implementation combines a cross-coupled quad cell and a source-coupled differential pair, as well as the current mirror technique in the output stage. As a result, improved linearity of the developed OTA over the large tuning range is obtained. SPICE simulations show that for 0.5-μm HP AMOS14TB process (MOSIS) with a 2.5 V power supply, total harmonic distortion (THD) at 2.5V_pp is less than 0.5%. The dynamic range is equal to 76 dB at power consumption 5.7 mW. The alternative OTA structures are also discussed and compared to the basic version; for this circuit THD at 2.5V_pp is less than 0.2%, however, the circuit is much more sensitive to the transistor mismatch. As an example, the OTA is used to design a third-order elliptic lowpass filter in the high-frequency range. The cutoff frequency of the filter is tunable in the range of 4.6-29.6 MHz