On the exact mid-band gain of the CMOS OTA with low-voltage-cascode-current-mirror

Abstract

This article presents an exact mid-band gain-expression for the CMOS operational-transconductance-amplifier (OTA) with low-voltage-cascode-current-mirror (LVCCM) load. Its small-signal analysis is not available in any CMOS text-book or other published sources/articles. A simplified and innovative technique is employed in performing this analysis with an in depth tutorial flavor. It shows that the derived mid-band gain of the CMOS OTA with LVCCM load is significantly more accurate than that obtained using the highly approximate classical gain-expression of the OTA employing a standard cascode-current-mirror equivalence which may be used in textbooks and other articles. Circuit simulations employing the 180-nm and 65-nm TSMC CMOS process technologies has been performed to indicate the progressively higher accuracy of the derived exact gain-expression for the LVCCM loaded OTA with technology scaling. This new analysis indicates an additional degree of freedom in tuning the gain of the OTA with LVCCM load using an additional transconductance parameter, gm5,6 of (M5, M6). A simplified expression is also deduced which provides an improvement over the classical approximate expression, due to the additional design insight and accuracy provided by this additional parameter while still being amenable to first order gain evaluation in a simplified way. The presented analysis is new and has not been reported before in open literature in any form.