Ideally, if both inputs of an operational amplifier (op-amp) are at exactly the same voltage, then the output should be at zero volts. In practice, a small differential voltage is applied to the inputs to cause the output to be zero volts. This differential voltage is known as the input offset voltage. The input offset voltage can be modelled as a voltage source in series with the inverting terminal of the op-amp and providing a constant voltage VOS.
Chopped amplifiers can be used to reduce or eliminate the effects of the input offset voltage VOS. A chopped amplifier can include, for example, an op-amp. Chopped amplifiers can operate in two phases in which a chopper selectively reverses the order of first and second differential inputs as outputs responsive to a control signal.
U.S. Pat. No. 6,380,801, the contents of which are incorporated herein by reference for all purposes, describes an operational amplifier having two differential input stages.
Johan H. Huijsing et al., “Low-Voltage Operational Amplifier with Rail-to-Rail Input and Output Ranges,” IEEE Journal of Solid-State Circuits, vol. sc-20, No. 6., December 1985, pp. 1144-1150, the contents of which are incorporated herein by reference for all purposes, describes an operational amplifier that can perform signal operations in nearly the full supply voltage range.
John A. Fisher et al., “A Highly Linear CMOS Buffer Amplifier,” IEEE Journal of Solid-State Circuits, vol. sc-22, No. 3, June 1987, pp. 330-334, the contents of which are incorporated herein by reference for all purposes, describes a CMOS buffer amplifier that achieves improvements in linearity and drive capability over previously reported high-swing amplifiers.
M. D. Pardoen et al., “A Rail-to-Rail Input/Output CMOS Power Amplifier,” IEEE Journal of Solid-State Circuits, vol. 25, No. 2, April 1990, pp. 501-504, the contents of which are incorporated herein by reference for all purposes, describes a rail-to-rail amplifier that maintains a high common-mode rejection ratio (CMRR) over the whole common-mode range and has a low harmonic distortion despite the use of relatively small output devices.