0.5-V bulk-driven differential ampliﬁer George Raikos* and Spyridon Vlassis 1 Electronic Laboratory, Department of Physics, University of Patras, GR-26504Rio Patras, Greece SUMMARY A new 0.5-V fully differential ampliﬁer is proposed in this article. The structure incorporates a differential bulk-driven voltage follower with conventional gate-driven ampliﬁcation stages. The bulk-driven voltage follower presents differential gain equal to unity while suppressing the input common-mode voltage. The ampliﬁer operates at a supply voltage of less than 0.5 V, performing input transconductance almost equal to a gate transconductance and relatively high voltage gain without the need for gain boosting. The circuit was designed and simulated using a standard 0.18-mm CMOS n-well process. The low-frequency gain of the ampliﬁer was 56 dB, the unity gain bandwidth was approximately 3.2 MHz, the spot noise was 100 nV/ √Hz at 100 kHz and the current consumption was 90 mΑ. Copyright © 2012 John Wiley & Sons, Ltd. Received 27 January 2011; Revised 27 March 2012; Accepted 8 April 2012 KEY WORDS: analog circuits; low voltage; ampliﬁers; bulk-driven transistors 1. INTRODUCTION During the last decades, the increasing applications of portable devices and the rapid scaling of VLSI process require the reduction of the supply and threshold voltages. Unfortunately, the supply voltages and threshold voltages are not scaled down by the same amount [1–3]. The supply voltage is related to the device reliability, whereas the threshold voltage is mainly driven by the digital VLSI speciﬁcations, such as speed, area, power dissipation, leakage currents or noise margin. For analog VLSI, the relatively large value of the threshold voltage, with respect to low supply voltage, is the main limitation in the implementation of low-voltage CMOS circuits. Thus, alternative analog design techniques and topologies capable for low supply voltages have emerged to achieve large signal-to- noise ratios and small nonlinearity. In the last years, the bulk-driven approach has been proposed in the implementation of extremely low voltage analog circuits [3–16]. On the basis of this approach, the input signal, that is applied to the bulk terminal of MOS devices, results in a large input common-mode range while an appropriate gate overdrive voltage maintains the device in the conductance region. The main limitations of this approach are the small bulk transconductance that is four to ﬁve times smaller than the gate-transconductance and, consequently, the relatively large input-referred noise compared with that of a gate-driven MOS device. The works of Blalock et al. [3], Grech et al. [4], and Layton et al. [5] report differential input pairs with tail current source or current mirrors, based on bulk-driven devices. The implemented ampliﬁers that are capable for sub-1-V supplies need gain boosting to compensate the limited differential gain (due to small bulk transconductance) [5,6]. In the ampliﬁer of Carrillo et al. [7], the input signal is handled again by a bulk-driven differential pair with tail current source; the voltage gain is enhanced using a partial positive feedback loop, and it operates at a supply voltage less than 1 V. The same *Correspondence to: George Raikos, Electronic Laboratory, Department of Physics, University of Patras, GR-26504, Rio Patras, Greece. E-mail: graikos@upatras.gr Copyright © 2012 John Wiley & Sons, Ltd. INTERNATIONAL JOURNAL OF CIRCUIT THEORY AND APPLICATIONS Int. J. Circ. Theor. Appl. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/cta.1820