Low distortion amplifier output stage for DAC

Abstract

A low distortion output stage for a digital-to-analog converter that operates with a switched-capacitor filter is provided that drives a low output impedance load. A source follower transistor (266) drives an output node (260) and a load resistor (264). A first current source is provided by transistor (280) to provide a constant drain current to transistor (266). A transistor (282) provides a current source between the output node and low reference voltage. A first current source is provided by transistor (284) to source current to resistor (264) during positive output voltage swings and a current source provided by the transistor (286) sinks current from resistor (264) during low voltage swings. The current through transistor (284) is controlled to operate in class AB by a P-channel transistor (288) and a bi-polar transistor (290). Transistor (290) has the gate thereof connected to the drain of transistor (266), as does transistor (284). As the voltage on the drain node (268) decreases, the current through transistor (284) increases, resulting in a decrease in current through transistor (288), which is mirrored to transistor (286) through transistor (292). When the gate voltage rises on transistor (284), the current increases through transistor (288) and increases current through transistor (286). The transistors (284) and (286) provide a low current to load resistor (264) that is independent of the drain current through transistor (266), thus resulting in distortion free operation.

Dependencies are inferred from the claim text.