An improved biasing circuit for high input CMR op amps

This paper introduces a biasing scheme which overcomes the inherent drawbacks associated with high input common-mode range (CMR) amplifiers: non-constant transconductance (G_m) and very poor common-mode rejection ratio (CMRR). The constant G_m is achieved by maintaining a constant sum of the square-roots of the complementary input pairs bias currents, while the high rejection to input common-mode signals is achieved by making a weak dependence of these currents as function of the input common-mode component. Simulated results show a maximum transconductance deviation less than 2% from its value for a common-mode input voltage at midsupply as well as a CMRR improvement over 15 dB respect to the classical biasing scheme