Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 3201 LECTURE 320 DIFFERENTIAL OUTPUT OP AMPS (READING: AH 384393, GHLM 808857) Objecive The objecive of his presenaion is: 1.) Design and analysis of differenial oupu op amps 2.) Examine he problem of common mode sabilizaion Ouline Advanages and disadvanages of fully differenial operaion Six differen differenial oupu op amps Techniques of sabilizing he common mode oupu volage Summary ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002 Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 3202 Why Differenial Oupu Op Amps? Cancellaion of common mode signals including clock feedhrough Increased signal swing v 1 v 2 A A A A Cancellaion of evenorder harmonics Symbol: v 1 v 2 2A 2A Fig. 7.31 v in v ou Fig. 7.31A ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002
Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 3203 Common Mode Oupu Volage Sabilizaion If he common mode gain no small, i may cause he common mode oupu volage o be poorly defined. Illusraion: v od 0 v od CM oupu volage = 0 0 v od 0 CM oupu volage =0.5 CM oupu volage =0.5 Fig. 7.32 ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002 Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 3204 TwoSage, Miller, DifferenialIn, DifferenialOu Op Amp V BP v i1 V BN Fig. 7.33 Oupu common mode range (OCMR) = V SDP (sa) V DSN (sa) The maximum peakopeak oupu volage 2 OCMR Conversion beween differenial oupus and singleended oupus: v id v od Fig. 7.34 C L v id 2C L 2C L ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002
Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 3205 DifferenialOupu, FoldedCascode, ClassA Op Amp M15 v i1 vo1 R 1 R 2 M16 V Bias M17 OCMR = 2V SDP (sa) 2V DSN (sa) Fig. 7.35 ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002 Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 3206 TwoSage, Miller, DifferenialIn, DifferenialOu Op Amp wih PushPull Oupu V BP v i1 V BN Commens: Able o acively source and sink oupu curren Oupu quiescen curren poorly defined Fig. 7.36 ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002
Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 3207 TwoSage, Differenial Oupu, FoldedCascode Op Amp M20 M19 M15 R 1 v i1 v R o2 z Rz M18 M16 V Bias M17 Fig. 7.37A Noe ha he followers and are necessary for level ranslaion o he oupu sage. ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002 Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 3208 Unfolded Cascode Op Amp wih DifferenialOupus M21 M20 v i1 M19 R 2 M22 R 1 M15 M16 M17 M18 M23 VBias Fig. 7.38 ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002
Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 3209 CrossCoupled Differenial Amplifier Sage One of he problems wih some of he previous sages, is ha he quiescen oupu curren was no well defined. The following inpu sage solves his problem. i 1 i 2 v i1 V GS1 V SG3 v GS1 v GS2 v SG3 v SG4 V GS2 V SG4 ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002 i 2 i 1 Fig. 7.39 Operaion: Volage loop v i1 = V GS1 v GS1 v SG4 V SG4 = V SG3 v SG3 v GS2 V GS2 Using he noaion for ac, dc, and oal variables gives, v i1 = v id = (v sg1 v gs4 ) = (v sg3 v gs2 ) If M1 = M2 = =, hen half of he differenial inpu is applied across each ransisor wih he correc polariy. i 1 = g m1v id 2 = g m4v id 2 and i 2 = g m2v id 2 = g m3v id 2 Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 32010 Class AB, Differenial Oupu Op Amp using a CrossCoupled Differenial Inpu Sage M26 M25 M15 M27 R 2 v i1 M28 V Bias M21 M19 M17 Fig. 7.310 Quiescen oupu currens are defined by he curren in he inpu crosscoupled differenial amplifier. ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002 M22 M20 M18 R 1 M24 M23 M16
Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 32011 CommonMode Oupu Volage Sabilizaion M1 Commonmode feedback circui M2 i o1 (source) R o1 R o2 i o2 (source) i o1 (sink) R o3 R o4 i o2 (sink) Model of oupu of differenial oupu op amp Fig. 7.311 Operaion: M1 and M2 sense he commonmode oupu volage. If his volage rises, he currens in M1 and M2 decrease. This decreased curren flowing hrough R o3 and R o4 cause he commonmode oupu volage o decrease wih respec o. ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002 Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 32012 TwoSage, Miller, DifferenialIn, DifferenialOu Op Amp wih CommonMode Sabilizaion V BP v i1 Commens: Simple Unreferenced V BN Fig. 7.312 ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002
Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 32013 A Referenced CommonMode Oupu Volage Sabilizaion Scheme V ocm I ocm I 5 I 6 To correcion circuiry Fig. 7.313 Operaion: 1.) The desired commonmode oupu volage, V ocm, creaes I ocm. 2.) The acual commonmode oupu volage creaes curren I 5 which is mirrored o I 6. 3.) If M1 hrough are mached and he curren mirror is ideal, hen when I ocm = I 6 he acual commonmode oupu volage should be equal o he desired commonmode oupu volage. 4.) The above seps assume ha a correcion circuiry exiss ha changes he commonmode oupu volage in he correc manner. ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002 Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 32014 Common Mode Feedback Circuis Implemenaion of common mode feedback circui: MB I Bias V CM MC3 Commonmode feedback circui I C3 MC1 MC5 MC2A MC4 I C4 MC2B v 3 I 3 I 4 v 1 v 4 Selfresisances of M1 v 2 Fig. 7.313A This scheme can be applied o any differenial oupu amplifier. Cauion: Be sure o check he sabiliy of commonmode feedback loops, paricularly hose ha are conneced o op amps ha have a cascode oupu. The gain of he commonmode feedback loop can easily reach ha of a wosage amplifier. ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002
Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 32015 Common Mode Feedback Circuis Coninued The previous circui suffers in performance when he differenial oupu volage becomes oo large and one of he MC2AMC2B ransisors shus off. The following circui alleviaes his disadvanage: I Bias MC3 Commonmode feedback circui V CM I C3 MC1 MC4 I C4 MC2 v 3 I 3 I 4 R CM1 R CM2 v 1 v 4 v 2 MB MC5 Fig. 7.315New ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002 Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 32016 Exernal CommonMode Oupu Volage Sabilizaion Scheme for DiscreeTime Applicaions φ 1 φ 2 φ 1 v id CMbias m m φ 1 V ocm φ 1 φ 2 φ 1 Fig. 7.314 Operaion: 1.) During he φ 1 phase, boh m are charged o he desired value of V ocm and CMbias = V ocm. 2.) During he φ 2 phase, he m capaciors are conneced beween he differenial oupus and he CMbias node. The average value applied o he CMbias node will be V ocm. ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002
Lecure 320 Differenial Oupu Op Amps (3/27/02) Page 32017 SUMMARY Advanages of differenial oupu op amps: 6 db increase in signal ampliude Cancellaion of even harmonics Cancellaion of common mode signals including clock feedhrough Disadvanages of differenial oupu op amps: Need for common mode oupu volage sabilizaion Compensaion of common mode feedback loop Difficul o inerface wih singleended circuis Mos differenial oupu op amps are ruly balanced For pushpull oupus, he quiescen curren should be well defined Common mode feedback schemes include, Unreferenced Referenced ECE 6412 Analog Inegraed Circui Design II P.E. Allen 2002