EECS240 – Spring 2010Lecture 15: Common-Mode FeedbackElad AlonDept. of EECSEECS240 Lecture 15 2Problem with Common-Mode Vi+Vi-ItailILIL• What if IL< Itail/2?• Will capacitive feedback solve this?• Typical solution: Common-mode feedback• Sense CM at output• Adjust some knob to alter CMEECS240 Lecture 15 3Common-Mode SensingVi+Vi-ItailILILVCMRR• Simplest CM sensor: pair of resistors• Resistors load the OTA (reduce gain)• If make R large, get slow Vcmtracking• Is this a problem?EECS240 Lecture 15 4Sensing Scheme #2Vi+Vi-ItailILILVsense• Isolated CM sensing• Works reasonably well• But hard to use with wide swing amplifier outputEECS240 Lecture 15 5Capacitive SensingVi+Vi-ItailILILVCMCC• Capacitive sensing avoids DC loading• (still creates AC load though)• Needs to be reset to remove initial offset• Just like capacitive feedbackEECS240 Lecture 15 6Adjusting Common-Mode• Really only two knobs:• Knob A: adjust load current• Knob B: adjust tail currentEECS240 Lecture 15 7Example Common-Mode FeedbackVi+Vi-+-Vcm_refVcmVbp• Secondary amplifier enforces Vcm= Vcm_ref• Place dominant pole at Vbp, or Vcm?EECS240 Lecture 15 8CMRR Fix• What if two PMOS transistors aren’t perfectly matched?EECS240 Lecture 15 9Capacitive CMFB• How to choose Ccm?• “Small”: CM loop gain low• “Large”: Loading on diff. output highEECS240 Lecture 15 10InitializationEECS240 Lecture 15 11“Continuous”
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