Extension of the common-mode range of bipolar input stages beyond the supply rails of operational amplifiers and comparators

Abstract

When the input voltage of an operational amplifier or comparator with a bipolar input stage exceeds the range of normal operation, the polarity of the output signal reverses and the input bias current increases to excessively large values. Saturation of the input transistors restricts the sensing of differential voltages to a common-mode (CM) range roughly between the positive and the negative supply rails. Input stage configurations that not only provide solutions to prevent the signal reversal and the excessive increase of input bias current, but also provide an extension of the CM range far beyond the supply rails, while the transconductance for differential input voltages remains constant, are described. Integrated implementations of the input stages realized a CM range reaching +15 V at a single supply voltage as low as 1 V, while the input bias current was limited to 6 mu A. >