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This paper proposes topological enhancements to increase voltage gain of ultra-low-voltage (ULV) inverter-based OTAs. The two proposed improvements rely on adoption of composite transistors and forward-body-biasing. The impact of the proposed techniques on performance figures is demonstrated through simulations of two OTAs. The first OTA achieves a 39 dB voltage gain, with a power consumption of 600 pW and an active area of 447 μm2. The latter allies the forward-body-bias approach with the benefit of the independently biased composite transistors. By combining both solutions, voltage gain is raised to 51 dB, consuming less power (500 pW) at the cost of an increased area of 727 μm2. The validation has been performed through post-layout simulations with the Cadence Analog Design Environment and the TSMC 180 nm design kit, with the supply voltage ranging from 0.3 V to 0.6 V.

This paper deals with a single-stage single-ended inverter-based Operational Transconductance Amplifiers (OTA) with improved composite transistors for ultra-low-voltage supplies, while maintaining a small-area, high power-efficiency and low output signal distortion. The improved composite transistor is a combination of the conventional composite transistor and forward-body-biasing to further increase voltage gain. The impact of the proposed technique on performance is demonstrated through post-layout simulations referring to the TSMC 180 nm technology process. The proposed OTA achieves 54 dB differential voltage gain, 210 Hz gain–bandwidth product for a 10 pF capacitive load, with a power consumption of 273 pW with a 0.3 V power supply, and occupies an area of 1026 μm2. For a 0.6 V voltage supply, the proposed OTA improves its voltage gain to 73 dB, and achieves a 15 kHz gain–bandwidth product with a power consumption of 41 nW.

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Inverter-based Operational Transconductance Amplifiers (OTAs) are versatile and friendly scalable analog circuit blocks. Especially for the new CMOS technological nodes, several recent applications have been extensively using them, ranging from Analog Front End (AFE) to analog-to-digital converters (ADC). This work tracks down the current advances in inverter-based OTAs design, comparing their basic fully differential structures, such as Nauta (N), Barthelemy (B), Vieru (V) and Mafredini (M) ones, and, in addition, mixing them up to propose new fully differential single-ended and two-stage hybrid versions. The new herein-proposed fully differential hybrid OTAs are the composition of Barthelemy/Nauta (B/N), Barthelemy/Manfredini (B/M), Nauta/Vieru (N/V), and Manfredini/Vieru (M/V) OTAs. All OTAs were designed using the same Global Foundries 180 nm open-source PDK and their performances are compared for post-layout simulations.