Design of digital OTAs with operation down to 0.3 V and NW power for direct harvesting

In this paper, passive-less fully-digital operational transconductance amplifiers (DIGOTA) for energy- and area-constrained systems are modeled and analyzed from a design viewpoint. The digital behavior of DIGOTAs is modeled as an equivalent small-signal differential-mode circuit with zero bias current, and a common-mode feedback loop operating as a self-oscillating threshold sampler. Such continuous-time equivalent circuits are used to derive an explicit model of the main performance parameters that are generally adopted to characterize OTAs. This provides an insight into circuit operation and allows to derive practical guidelines to achieve a given design target. Among the others, an explicit model is derived for the DC gain, the frequency response, the gain-bandwidth product, the input-referred noise, and the input offset voltage. The models are validated via direct comparison with multi-die measurement results in CMOS 180 nm. From an application viewpoint, the voltage (power) reduction down to 0.25 V (subnW) uniquely enable direct harvesting (e.g., with solar cells), suppressing any intermediate DC-DC conversion stage. This further enhances the area efficiency advantage of DIGOTA stemming from its fully-digital nature, making it well suited for cost-sensitive and purely-harvested systems.