7 Two-Stage, Differential Output, Folded-Cascode M11-M13 and M10-M12 provide level shift
8 Common Mode Output Voltage Stabilization Common mode drift at output causes differential signals move into triode region
9 Common Mode feedback All fully differential amplifier needs CMFB Common mode output, if uncontrolled, moves to either high or low end, causing triode operationWays of common mode stabilization:external CMFBinternal CMFB
10 Cause of common mode problem Unmatched quiescent currentsVbb=VbbQ+ΔVbbI2Vin=VinQVbb=VbbQVo1Vo2VinI1Vo1QVo1Vin=VinQ+ΔVinactual Q point M2 is in triode
12 Basic concept of CMFB: Dvb e desired common mode voltage CM measurementVo+ +Vo-2Vo+Vo-Voc-CMFBDvbeVoCM+desired common mode voltage
13 Basic concept of CMFB: Dvb e e measurementVo+ +Vo-2Vo+Vo-Voc-CMFBDvbeeVoCM+Find transfer function from e to Voc, ACMF(s)Find transfer function from an error source to Voc Aerr(s)Voc error due to error source: err*Aerr(0)/ACMF(0)
25 To increase or decrease the C.M. loop gain: e.g.Vo.c.Vo.c.d.Vo.c.Vo.c.d.VC.M.F.B.VC.M.F.B.
26 Another implementation Use triode transistors to provide isolation & z(s) simultaneously.M1, M2 in deep triode.VGS1, VGS2>>VTVocVo+Vo-M1M2In that case, circuit above M1, M2 needs to ensure that M1, M2 are in triode.can be a c.s.
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