  A strain sensor inspired by a Widlar self-biased current source topology
called $\beta$-multiplier is developed to obtain a strain-dependent reference
current with high supply rejection. The sensor relies on the piezoresistive
effect in the silicon MOS transistors that form the current reference circuit.
The device behavior is analytically computed and verified with experimental
measurements under four-point bending test. A basic implementation with an
integrated resistor reaches a strain sensitivity of 2.54 nA/$\mu\epsilon$
(gauge factor of 324) for a temperature sensitivity of 52.06 nA/{\deg}C. A more
advanced full-transistor circuit based on current subtraction principle is
furthered implemented in order to reach strain sensitivity up to 12.02
nA/$\mu\epsilon$ (gauge factor of 1773) and temperature sensitivity of -28.72
nA/{\deg}C. This implementation includes a CMOS active load to tune the strain
and temperature sensitivities with a total power consumption between 20 and 150
$\mu$W.
