1. Field of the Invention
This invention relates generally to operational amplifiers, and, more particularly, to means for improving the common mode (CMR) and power supply (PSR) rejection characteristics of such amplifiers.
2. Description of the Related Art
Operational amplifiers typically include an input stage built around a differential transistor pair biased with a tail current. The tail current, as well as other input stage bias currents, are typically provided with respective transistors, each of which is controlled with a drive voltage provided by a bias generator circuit.
However, the output impedances of the bias current transistors may not be particularly high. As such, variations in the amplifier's supply voltage and in the input common-mode voltage can cause the bias currents to vary, which serves to degrade the amplifier's CMR and PSR characteristics.
One method by which CMR and PSR can be improved is by connecting a cascode transistor or an output impedance boost amplifier in series with the bias current transistors; cascode transistors can also be added within the bias generator and/or the input stage. However, these added components require additional headroom, thereby increasing the amplifier's minimum supply voltage.
Many other biasing methods are known; several are described, for example, in “Analysis and Design of Analog Integrated Circuits”; Gray and Meyer, 3rd ed., pp. 310-311, 322-333, 422. However, most of these methods also exhibit less than ideal CMR/PSR characteristics, or require a considerable amount of headroom.