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 CMR (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 mum standard CMOS technology with supply voltage of 1.8 V. We could demonstrate a maximum of upto 40 dB increase in CMRR for a range of common-mode voltages.