The design of differential pair based OTAs is becoming increasingly difficult in finer geometries due to lower supply voltages. Inverter based designs have proven to have better transconductance efficiency, higher swing and better linearity but have degraded CMRR, worse PSRR and limited PVT tolerance. In this tutorial, we discuss traditional amplifiers and why inverter based amplifiers are better suited for lower supplies. We then describe the design procedure for inverter based OTA designs with an emphasis on improving their performance, including PVT tolerance, CMRR and PSRR. In particular, we introduce new biasing techniques for inverters to improve their PVT tolerance. We finally validate our designs using measurement results from a number of fabricated designs.