Low-voltage high CMRR OTA for electrophysiological measurements

With the advent of miniaturised sensors for various engineering and medical applications there is an increased demand of low-power, low-voltage analog building blocks like opamps and OTAs. Degradation of certain amplifier characteristics with supply voltage is a major concern for low-voltage design and often poses contradictory requirement. CMRR (common mode rejection ratio), one such feature, is essential for sensing small bio-electric signals riding over large common-mode voltage. Here we focus on the challenging task of building a low-voltage OTA with high CMRR along with wide CMRR (common mode range) and high output impedance. Robust tail current sources for a complementary differential pair OTA with folded cascode output is implemented for the purpose. The circuits are designed and simulated in 0.35 mu m standard CMOS technology with supply voltage of 1.8V. We could demonstrate a maximum of upto 40dB increase in CMRR for a range of common-mode voltages.