${\mathrm{V}}_{\text {DD }}$-Scalable and Reference-Less Body-Biased Inverter-Based OTA Exploiting Improved Composite Transistors for ULV/ULP Applications

This paper deals with single-stage, single-ended inverter-based Operational Transconductance Amplifiers (OTAs) designed with Rectangular Transistor Arrays (RTAs) and Improved Composite Transistors (ICTs), targeting ultra-low-voltage and ultra-low power applications. Both solutions are implemented using 180nm CMOS standard process technology and validated through experimental results. Both the OTAs maintain compact size (typical of a digital-based architecture), enhanced average slew rate, $S R_{a v}$, performance, and, hence, improved large-signal power efficiency (high FOML). The proposed ICT solution exploits forward-body biasing and composite transistor architecture to improve the differential voltage gain compared to the RTA implementation of the same OTA. Operations from a supply voltage, VDD, ranging from 0.5 V to 0.3 V are demonstrated in this work. In particular, the proposed reference-less inverter-based ICT OTA achieves a differential voltage gain of $50 \text{dB}(35 \text{dB})$ and 17 kHz $(0.3 \text{kHz})$ gain-bandwidth product, $G B W$, at $0.5 \mathrm{V}(0.3 \mathrm{V})$, while driving $30-\text{pF}$ capacitive load. It shows power consumption ranging from 50 nW at 0.5 V down to only 750 pW at 0.3 V, while occupying $3,250 \mu \mathrm{m}^{2}$.