**1. Introduction**

The effort to develop implantable or bio-sensing battery-less biomedical instrumentation systems has been continuously challenging analog designers because of the intensified constraints arising from CMOS scaling [1–3]. Topological solutions for endowing operational transconductance amplifiers (OTAs) to process μV signals with common-mode swings in the range of tens of volts, allied to features like ultra-low power consumption, low-noise, enhanced linearity, high common-mode rejection ratio (CMRR), tiny silicon footprint, and large common-mode range (CMR) are frequently pursued by the analog circuit designers [4–18].

As a basic block in analog front-ends (AFEs) for biosensing, the OTA-C filter with large time constants is among the most important applications for OTAs with reduced transconductance [19]. Such circuits when used in implantable/wearable biomedical applications have their design challenged by the restricted-sized on-chip integrated capacitors. In order to decrease the size of such filters, OTAs must output a very small transconductance in the order of a few nA/V, which is achieved with very low biasing currents [20] at the cost of the OTA linearity.

Among the typical OTA design techniques to increase linearity is the use of nonunity gain current mirrors [21–23] to allow higher biasing currents and maintain a low transconductance. Another well-known technique that is used to improve both OTA linearity and input signal voltage swing is the bulk-driven differential pair [1,24–29]. Unlike the gate-driven OTA topologies, the bulk-driven OTAs outputs are an alternative for a relatively lower transconductance [20,30]. In this case, the main drawback of this approach is a poor DC voltage gain, which can be improved by using several techniques [25]. An interesting and widely employed technique relies on a self-cascode topology known

**Citation:** Sanchotene Silva, R.; Rodovalho, L.H.; Aiello, O.; Ramos Rodrigues, C. A 1.9 nW, Sub-1 V, 542 pA/V linear Bulk-Driven OTA with 154 dB CMRR for Bio-Sensing Applications. *J. Low Power Electron. Appl.* **2021**, *11*, 40. https://doi.org/ 10.3390/jlpea11040040

Academic Editor: Stylianos D. Assimonis

Received: 1 August 2021 Accepted: 17 October 2021 Published: 20 October 2021

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as trapezoidal or composite transistor [31–34]. Additionally, an improvement for the selfcascode transistor association was proposed in [13,35–37] allowing to increase voltage gain and decrease area usage. Therefore, in this paper, we propose a new symmetrical bulkdriven OTA topology that takes the advantages of previously described techniques, i.e., the combination of the topology presented by [23], with a bulk-driven differential pair [24], and the bulk-driven active source degeneration linearization technique adapted from [1,38]. Besides the employed combination of techniques in the OTA topology, we propose an innovative improved self-cascode current mirror (ISCCM) which is based on [35,37].

This paper is organized as follows: Section 2 describes current mirror topologies made of rectangular transistor arrays (composite transistor). The improved self-cascode current mirror that sources the proposed OTA is introduced. Section 3 presents the bulk-driven symmetrical OTAs topologies. Simulations and comparisons among the proposed BD topology, the conventional bulk-driven, and state-of-art transconductors are shown in Section 4. Finally, Section 5 presents the conclusions.
