Calibration of the Nauta Structure Differential OTA

This work identifies properties and behaviours of a differential operational transconductance amplifier (OTA) consisting of six inverters, first proposed by Bram Nauta, that are exploited to design a digital calibration technique for a modified, digitally-controllable version of the OTA. The calibration technique is designed to reliably tune the OTA for high gain in such a way that very little additional analogue hardware is required, that then could be implemented as an integrated on-chip solution.Due to its simple topology the Nauta structure is suitable for operation with low supply voltages and can have a high bandwidth, while still exhibiting a high gain. Such properties make the Nauta structure exceptionally well suited to being scaled down and implemented in deep sub-micron technologies. Unfortunately the Nauta structure is also highly susceptible to device mismatch which is a major issue for deep sub-micron technologies and this shortcoming must be addressed to ensure the utility of the Nauta structure. To deal with the issue of mismatch a modified, digitally controllable Nauta structure is used. In this structure, conventional inverters are replaced with digital-to-transconductance (DTC) converters, whose transconductances can be controlled with a digital control code, this in turn allows the Nauta structure to be tuned for high gain and mitigate the effects of mismatch.In the Nauta structure the gain is enhanced by the presence of positive feedback and excessive positive feedback is known to cause hysteresis. This work correlates the presence of hysteresis, or lack thereof and high gain. In this work a number of calibration approaches exploiting the presence of hysteresis are examined and their feasibility is discussed and investigated. Based on the gathered data, a synthesisable digital calibration algorithm is then presented