EE 435. Lecture 10: Folded-Cascode Amplifiers Current Mirror Op Amps

Transcription

1 EE 435 ecture 0: Folded-ascode mplifiers urrent Mirror Op mps

2 Where we are at: Basic Op mp Desin Fundamental mplifier Desin Issues Sinle-Stae ow Gain Op mps Sinle-Stae Hih Gain Op mps Other Basic Gain Enhancement pproaches Two-Stae Op mp

3 Review from last lecture: Hih output impedance quarter-circuits OUT - M 3 M Quarter ircuit Reulated ascode mplifier or Gain Boosted ascode is usually a simple amplifier, often the reference op amp with + terminal connected to the desired quiescent voltae ssume biased with a dc current source (not shown) at drain of M 3 3

4 Review from last lecture: Gain-Boosted Telescopic ascode Op mp (with or w/o current mirror counterpart circuits) dvantaes: DD Sinificant increase in dc ain M 5 M 6 M S S B B3 M 3 M 4 M M I T B5 M M 8 OU T imitations: : Sinal swin (4D ST + T between DD and SS ) Reduction in GB power efficiency - some current required to bias amplifiers -additional pole in amplifier -may add requirements for some compensation rea Overhead for 4 transistors and 4 amplifiers -actually minor concern since performance will usually justify these resources 4

5 Review from last lecture: re there other useful hih output impedance circuits that can be used for the quarter circuit? G o + d G M G 0 BW GB G G GM G G M G 5

6 M M Review from last lecture: Implementation of Biased Folded ascode mplifier? I B OUT DD DD B M 3 I B B M 3 M5 B3 SS Biased Folded ascode Implementation of Biased Folded ascode 6

7 nalysis of Biased Folded ascode B M M 3 M 5 DD B3 OUT o3 s m33 Xo3 X o o3 o5 m33 OUTo3 3 X IN OUT OUT o3 s m3 o3 3 0 IN 3 m3+o o3 o5 +OUTo3 OUT G IN o3 s o M o5 m3 o3 3 M3 3 o5 M5 5 5 X G O M o How can this be seen by inspection? First observe if all o s are 0, G M = Then observe M 3 cascodes the impedance 0 + o5 7

8 Biased Folded ascode Quarter ircuit OUT OUT IN s o o5 o3 m3 M 3 DD B B M5 0 m3 o o5 o3 M GB 8

9 Basic mplifier Structure omparisons (ideal current source biasin) ommon Source ascode Reulated ascode Folded ascode Small Sinal Parameter Domain 0 m m3 0 o o3 m3 0 o o3 0 o m3 o o5 o3 m GB m GB m GB GB m 9

10 Basic mplifier Structure omparisons Practical Parameter Domain ommon Source ascode Reulated ascode Θ=pct power in O O O 4 λ λ 4 λ λ 3 3 λ EB EB EB EB3 EB3 GB GB P DD DD P EB EB P - θ GB DD EB Folded ascode Θ=fraction of current of M 5 that is in M O 4θ θλ λ5 λ3eb EB3 GB P DD θ EB 0

11 Biased Folded-ascode mplifier DD B M 3 M5 B3 M M B M 4 M 6 B4 SS Quarter ircuit ounterpart ircuit

12 Folded-ascode Operational mplifier DD B5 B5 DD B B3 B B B3 SS SS OUT OUT DD DD B B B4 I T QURTER IRUIT SS Op mp

13 Folded-ascode Operational mplifier (redrawn) DD DD M 5 M 6 B B5 M 3 M 4 M M B5 OUT B OUT M M M7 M 9 B3 M 0 M 8 B4 I T I T SS SS These transistors pair-wise form a current source and one in each pair can be removed 3

14 Folded ascode Op mp DD M 5 M 6 B M 3 M 4 OUT B OUT M 9 M 0 M M M7 B3 M 8 B4 I T I T SS Needs MFB ircuit for B4 Either sinle-ended or differential outputs an connect counterpart as current mirror to eliminate MFB Foldin caused modest deterioration of 0 and GB enery efficiency Modest improvement in output swin 4 SS

15 SS I T M M M7 Folded ascode Op mp (Sinle-ended Output) 0 M 9 SS DD M 5 M 6 B M 3 M 4 B B3 I T M 0 M 8 OUT OEQ O3 O O5 m3 O7 s O9 m9 meq s meq OEQ O3 O 0 GB O5 meq OEQ m3 meq GB O7 5 O9 m9

16 Operational mplifier Structure omparison Small Sinal Parameter Domain Reference Op mp O O O3 GB IT SR Telescopic ascode o o o3 m3 o7 o5 m5 GB IT SR Reulated ascode o o o3 m3 o7 o9 m9 3 GB IT SR Folded ascode o o o5 o3 m3 o7 o9 m9 GB IT SR 6

17 Operational mplifier Structure omparison Practical Parameter Domain Reference Op mp 0 λ λ 3 EB GB P DD EB SR P DD Telescopic ascode Reulated ascode Θ=pct power in Folded ascode Θ=fraction of current of M 5 that is in M 0 0 EB λ λ 3 λ EB3 5 λ 7 EB5 0 P- θ λ λ3eb3 λ5λ7eb7 EB GB DD 3 EB θλ λ λ θ 5 3 θ EB3 λ 9 λ 7 EB9 GB GB P DD P DD θ EB EB EB SR P DD P - θ SR SR θ P DD DD 7

18 Folded ascode Op mp (Sinle-ended Output) DD M 5 M 6 B M 3 M 4 B OUT 0 O3 O O5 m3 GB O7 O9 m9 M 9 M M M7 B3 M 0 How many derees of freedom are there? M 8 What is a practical desin parameter set? I T I T DOF? 9 DOF {I T,W /,W 5 / 5,W 3 / 3,W 9 / 9,W 7 / 7, B, B, B3 } SS SS Practical Desin Parameters {P,θ, EB, EB3, EB5, EB7, EB9, B, B3 } where θ=i T /(I T +I T ) 8

19 Textbook reference: Some of the material we have been discussin appears in hapter 3, some in hapter 5, and some in hapter 6 of the Martin and Johns text In particular, the telescopic and folded cascode structures are referred to as advanced op amps and appear in later chapters of the text 9

20 Folded Gain-boosted ascode mplifier o o o3 m3 - I B M 3 OUT GB M I B with ideal current source bias modest improvement in output swin 0

21 Folded Gain-boosted ascode mplifier I BIS OUT IN 0 s o o5 o m3 m3 o5 o3 o3 - M DD OUT M 3 M5 B GB modest improvement in output swin

22 Basic mplifier Structure omparisons ommon Source ascode Reulated ascode Folded ascode Folded Reulated ascode Small Sinal Parameter Domain O O O O O O O m O m3 o3 m3 o3 m3 O O5 o3 O O5 m3 o3 GB m GB GB GB GB

23 Basic mplifier Structure omparisons ommon Source ascode Reulated ascode Θ=pct power in Folded ascode Θ=fraction of current of M 5 that is in M O O Practical Parameter Domain O O 4 λ λ 4 λ λ 3 λ 3 4θ EB EB EB EB3 EB3 θλ λ5 λ3eb EB3 GB GB GB P DD P DD P DD EB P - θ GB DD EB θ EB EB Folded Reulated ascode Θ =pct of total power in Θ =fraction of current of M 5 that is in M O 4θ θ λ λ5 λ3eb EB3 GB P DD θ - θ EB 3

24 Folded Gain-boosted Telescopic ascode Op mp DD o O O5 o3 3 m3 o7 o9 m9 B M 5 M 6 M M 4 B OU T B3 GB I N I N M M M7 M 9 M M 8 B4 0 I T I T S S S S Needs MFB ircuit for B4 Either sinle-ended or differential outputs an connect counterpart as current mirror to eliminate MFB Foldin caused modest deterioration in GB efficiency and ain Modest improvement in output swin 4

25 Operational mplifier Structure omparison Small Sinal Parameter Domain Reference Op mp O O O3 GB IT SR Telescopic ascode o o o3 m3 o7 o5 m5 GB IT SR Reulated ascode o o o3 m3 o7 o9 m9 3 GB IT SR Folded ascode o o o5 o3 m3 o7 o9 m9 GB IT SR Folded Reulated ascode o o o5 o3 m3 3 o7 o9 m9 9 GB IT SR 5

26 Summary of Folded mplifier Performance Modest improvement in output sinal swin (from 5 DS ST to 4 DS ST ) an directly feed output back to input to create buffer Deterioration in 0 (maybe 30% or more) Deterioration in GB power efficiency (can be sinificant) Minor increase in circuit size 6

27 Other Methods of Gain Enhancement BB Recall: DD ounterpart ircuit Quarter ircuit OUT 0 OQ GB MQ mq Two Strateies: O SS. Decrease denominator of 0. Increase numerator of 0 Previous approaches focused on decreasin denominator onsider now increasin numerator 7

28 Determination of op amp characteristics from quarter circuit characteristics Small sinal Quarter ircuit Small sinal differential amplifier DD I XX OUT OUT P BB OUT F d F d SS I BIS or SS Q ( s) GM s G G M O s G G d Note that the counterpart circuit is simply servin as the biasin current source ould use counterpart circuits (or other circuits) from other quarter circuits for P ounterpart circuits connected as one-port 8 an think of makin differential op amp directly from quarter circuit

29 Differential input op amp directly from quarter circuit DD I XX F OUT Q ( s) GM s G OUT d P F BB d OUT G M O d s G G IBIS SS or SS DD DD OUT I BB I BB OUT OUT I BB I BB OUT d F d d F d I BIS G M O s G G d I BB SS 9

30 meq Gain Enhancement Stratey I B MQ M m is increased by the mirror ain! : M OUT Foldin is required to establish the correct bias current direction onsider usin the quarter circuit itself to form the op amp M onsider this quarter circuit ould have done this for other quarter circuits as well but there is a particularly important reason we are followin this approach with this quarter circuit What is it? Output conductance of Q: OQ 30

31 meq Gain Enhancement Stratey : M MQ = M M OEQ OQ OI BB OUT M I B Redraw to absorb I B in the quarter circuit 3

32 meq Gain Enhancement Stratey : M M : : M OUT OUT OUT I B I B M I B M M I T 3

33 urrent Mirror Op mps OUT M : : M OUT I B I B - OUT 0 = + + IN - IN M M I T ery Simple Structure! Premise: Transconductance ain increased by mirror ain M OEQ OQ OI BB Premise: If output conductance is small, ain can be very hih Premise: GB very ood as well Still need to enerate the bias current I B M meq 0 GB meq (for += d /) meq OEQ 33

34 urrent Mirror Op mps DD DD M : : M M 5 M 3 M 4 M 6 OUT OUT OUT OUT SS M M SS M M SS I T B M 9 B I T M 7 M 8 SS SS SS Need MFB to establish B Basic urrent Mirror Op mp an use hiher output impedance current mirrors an use current mirror bias to eliminate MFB but loose one output 34

35 Is this a real clever solution? 35

36 Basic urrent Mirror Op mp OUT DD M 5 M 3 M 4 M 6 OUT M meq OEQ O6 O8 B M 9 B SS M M I T M 7 M 8 SS SS GB M M O O6 O8 MFB not shown SR MI T 36

37 urrent-mirror Op mp offers stratey for m enhancement ery Simple Structure Has applications as an OT Based upon small sinal analysis, performance appears to be very ood! But how ood are the properties of the MO? Is this a real clever solution? 37

38 Seminal Work on the OT From: N.E.. PROEEDINGS 38

39 Seminal Work on the OT From: 969 N.E.. PROEEDINGS December

40 Oriinal OT I B I M I I I I I I B I OUT I 3 I 4 Q Q I B Q 3 Q 4 urrent Mirror I I I B 4 I I I 3 OUT 3 I 4 I OUT M I B I 40

41 Oriinal OT I B I M I I I MI IB MI I I I I B I OUT I 3 I 4 Q Q I B Q 3 Q 4 I OUT M I B I 4

42 Oriinal OT I MI IB MI I I I I B I OUT I 3 I 4 Q Q I B Q 3 Q 4 I OUT M I B I 4

43 Oriinal OT I MI IB MI I I I I B I OUT I 3 I 4 Q Q I B Q 3 Q 4 3-mirror OT I OUT M I B I 43

44 urrent Mirror Op mp W/O MFB DD M : : M M meq OUT Often termed an OT I T I OUT SS m : SS Introduced by Wheatley and Whitliner in 969 I OUT m IN 44

45 End of ecture 0 45