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Common mode feedback for large output swing and low differential error

  • US 20080246543A1
  • Filed: 04/03/2007
  • Published: 10/09/2008
  • Est. Priority Date: 04/03/2007
  • Status: Active Grant
First Claim
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1. A differential amplifier comprising:

  • (a) a differential input transistor pair including first and second input transistors each having a first electrode, a second electrode, and a control electrode;

    (b) a first folded cascode stage including first, second, third and fourth current source transistors and first and second cascode transistors each having a first electrode, a second electrode, and a control electrode, first and second summing junctions coupled to the second electrodes of the first and second input transistors, respectively, and first and second outputs conducting first and second output signals, respectively, the second electrodes of the first and second current source transistors being coupled by the first and second summing junctions, respectively, to the first electrodes of the first and second cascode transistors, the second electrodes of the first and second cascode transistors being coupled to the first and second outputs, respectively, and the second electrodes of the third and fourth current source transistors being coupled to the first and second outputs, respectively; and

    (c) common mode feedback circuitry including(1) a tracking circuit coupled to the first and second outputs for producing first and second common mode output signals in response to the first and second output signals, and(2) an auxiliary amplifier stage including a differential input stage including third, fourth and fifth input transistors, gates of the third and fourth input transistors being coupled to receive the first and second common mode output signals, respectively, a gate of the fifth input transistor being coupled to receive a desired common mode voltage, drains of the third and fourth input transistors being coupled to a first input of a secondary amplifying stage having an output, a drain of the fifth input transistor being coupled to a second input of the secondary amplifying stage, the output of the secondary amplifying stage coupling a common mode feedback control signal to bias the control electrodes of the third and fourth current source transistors.

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