Low-voltage, low-power Vt independent voltage reference for bio-implants

Biomedical electronics trends focus mainly on portability, miniaturization, connectivity, humanization, security and reliability. In this scenario, digital, low-cost CMOS technology plays a key role, especially in implementing complex systems into small devices with no batteries that can even be implanted in humans. Due to patient safety, the implanted devices are faced with challenges: device operation temperature and the RF power link must be kept extremely low.By using proper topologies, the whole system can be designed to operate in low-voltage and low-power modes to maintain low temperature and avoid tissue thermal hazards. In this paper, a voltage reference is proposed which can operate at as low as 500 mV with power consumption less than 100 nW. Furthermore, the proposed topology, based on composite transistors operating in weak inversion, shows a good rejection to threshold voltage Vt, which is an inherent CMOS dispersion parameter. Simulation results using the process corners show that the Vt dependence can be reduced to less than ±2% (3σ) at the body temperature and the PSRR can be as large as 65 dB for higher frequencies. One of the key features of the circuit is its simple design.