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林維豪 2005. 5.25

CMFB. 林維豪 2005. 5.25. Outline. Introduction of CMFB Some structure of CMFB. Outline. Introduction of CMFB Some structure of CMFB. Why CMFB. When use fully differential, we need Common mode feedback Output : Define the output common mode voltage

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林維豪 2005. 5.25

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  1. CMFB 林維豪 2005. 5.25

  2. Outline • Introduction of CMFB • Some structure of CMFB

  3. Outline • Introduction of CMFB • Some structure of CMFB

  4. Why CMFB • When use fully differential, we need Common mode feedback • Output : Define the output common mode voltage • Input: With DC variation, stable the output common mode voltage • Suppress Acm-dm

  5. The basic structure of CMFB Treated as small signal

  6. Small signal model • CMFB bandwidth should be larger than differential mode

  7. Outline • Introduction of CMFB • Some structure of CMFB

  8. Sense Amplifier • Advantage : Linear sense • Disadvantage : Loading effect • Advantage : Linear sense • Disadvantage : Limit swing

  9. Error Amplifier • Advantage : High Gain • Disadvantage :1.Limit swing 2.Suffer non-linearity and mismatch

  10. Exmple

  11. Voltage control resistor • Advantage : Light loading effect • Disadvantage :1.Limit swing 2.Small gain

  12. Low voltage

  13. Switch capacitor • Advantage : 1.No swing limitation 2. Avoid R loading effect • Disadvantage :1.Longer settling time 2.Increase C output load 3.Need clock

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