  The robustness of current and voltage references to process, voltage and
temperature (PVT) variations is paramount to the operation of integrated
circuits in real-world conditions. However, while recent voltage references can
meet most of these requirements with a handful of transistors, current
references remain rather complex, requiring significant design time and silicon
area. In this paper, we present a family of simple current references
consisting of a two-transistor (2T) ultra-low-power voltage reference, buffered
onto a voltage-to-current converter by a single transistor. Two topologies are
fabricated in a 0.18-$\mu$m partially-depleted silicon-on-insulator (SOI)
technology and measured over 10 dies. First, a 7T nA-range
proportional-to-absolute-temperature (PTAT) reference intended for
constant-$g_m$ biasing of subthreshold operational amplifiers demonstrates a
0.096-nA current with a line sensitivity (LS) of 1.48 %/V, a temperature
coefficient (TC) of 0.75 %/$^\circ$C, and a variability $(\sigma/\mu)$ of 1.66
%. Then, two 4T+1R $\mu$A-range constant-with-temperature (CWT) references with
(resp. without) TC calibration exhibit a 1.09-$\mu$A (resp. 0.99-$\mu$A)
current with a 0.21-%/V (resp. 0.20-%/V) LS, a 38-ppm/$^\circ$C (resp.
290-ppm/$^\circ$C) TC, and a 0.87-% (resp. 0.65-%) $(\sigma/\mu)$. In addition,
portability to common scaled CMOS technologies, such as 65-nm bulk and 28-nm
fully-depleted SOI, is discussed and validated through post-layout simulations.
