Extension of the Input Common-Mode Range Beyond the Supply Rails of Operational Amplifiers and Comparators

When the input common-mode (CM-) voltage of an Operational Amplifier or Comparator exceeds the range of normal operation the (bipolar) input transistors saturate. This results in reversal of the signal polarity at the output, and in an increasing input bias current. These saturation effects cause very undesirable OpAmp behaviour. Satisfactory solutions for these problems are not found in existing input stage configurations. Novel circuit topologies for input stages with an extended CM-range are presented. The designed and realised input stages do not show the above-described adverse effects, and offer the advantage of an input CM-range extending far beyond the supply rails. The transconductance of these new input stages is constant over a full (extended) CM-range, and the input bias current is limited. A realisation of one proposed input stage configuration showed a CM-range that reached +14 Volt, at a supply voltage down to 1.3 Volt, while the input bias current was limited to 2.5 uA.