EE 435. Lecture 10: Current Mirror Op Amps

Transcription

1 EE 435 ecture 10: Current Mirror Op mps 1

2 Review from last lecture: Folded Cascode mplifier DD DD B3 B3 B1 B3 B B B3 DD DD B1 B1 B4 I T QURTER CIRCUIT Op mp

3 Review from last lecture: Folded Cascode Op mp DD M 5 M 6 B1 M 3 M 4 B M 9 M 10 M 1 M M7 B3 M 8 B4 I T I T Needs CMFB Circuit for B4 Either sinle-ended or differential outputs Can connect counterpart as current mirror to eliminate CMFB Foldin caused modest deterioration of 0 and GB enery efficiency Modest improvement in output swin 3

4 Review from last lecture: Folded Gain-boosted Cascode mplifier o o1 o3 m3 - I B1 M 3 GB C M 1 I B with ideal current source bias modest improvement in output swin 4

5 Review from last lecture: Basic mplifier Structure Comparisons Common Source Cascode Reulated Cascode Folded Cascode Folded Reulated Cascode Small Sinal Parameter Domain O O O O O O1 O1 m O m3 o3 m3 o3 m3 O1 O5 o3 O1 O5 m3 o3 GB C m GB C GB C GB C GB C 5

6 Review from last lecture: Basic mplifier Structure Comparisons Common Source Cascode Reulated Cascode Θ=pct power in Folded Cascode Θ=fraction of current of M 5 that is in M 1 O O Practical Parameter Domain O O 4 λ λ 1 4 λ λ 1 3 λ 3 4θ 1 EB1 EB EB1 1 EB3 EB3 θλ 1 λ5 λ3eb1 EB3 P GB DDC P GB DDC P GB DDC 1 EB 1 EB1 P 1- θ GB DD C θ EB1 EB1 Folded Reulated Cascode Θ 1 =pct of total power in Θ =fraction of current of M 5 that is in M 1 O 4θ θ λ1 λ5 λ3eb1 EB3 P GB DDC θ 1- θ EB1 1 6

7 Review from last lecture: Folded Gain-boosted Telescopic Cascode Op mp DD o O1 O5 o3 3 m3 o7 1 o9 m9 B1 M 5 M 6 M M 4 B OU T B3 GB C I N I N M 1 M M7 M 9 1 M 1 M 8 B4 0 C I T I T S S Needs CMFB Circuit for B4 Either sinle-ended or differential outputs Can connect counterpart as current mirror to eliminate CMFB Foldin caused modest deterioration in GB efficiency and ain Modest improvement in output swin Can directly connect as unity ain buffer S S 7

8 Review from last lecture: Operational mplifier Structure Comparison Small Sinal Parameter Domain Reference Op mp O 1 O1 O3 C GB IT SR C Telescopic Cascode o o1 o3 m3 o7 o5 m5 C GB IT SR C Reulated Cascode o o1 o3 m3 1 o7 o9 m9 3 C GB IT SR C Folded Cascode o o1 o5 o3 m3 o7 o9 m9 C GB IT SR C Folded Reulated Cascode o o1 o5 o3 m3 3 o7 o9 m9 9 C GB IT SR C 8

9 Other Methods of Gain Enhancement BB Recall: DD Counterpart Circuit Quarter Circuit C 0 OQC GB MQC mqc C Two Strateies: OCC 1. Decrease denominator of 0. Increase numerator of 0 Previous approaches focused on decreasin denominator Consider now increasin numerator 9

10 Determination of op amp characteristics from quarter circuit characteristics Small sinal Quarter Circuit Small sinal differential amplifier DD I XX OUT P BB OUT F C C d F d C I BIS or QC ( s) GM sc G G M1 O sc G G d 1 Note that the counterpart circuit is simply servin as the biasin current source Could use counterpart circuits (or other circuits) from other quarter circuits for P Counterpart circuits connected as one-port 10 Can think of makin differential op amp directly from quarter circuit

11 Differential input op amp directly from quarter circuit DD I XX F C QC ( s) GM sc G OUT C d P F BB d OUT C G M1 O d sc G1 G IBIS or SS DD DD OUT I BB I BB OUT OUT I BB I BB OUT C d F d C C d F d C I BIS G M1 O sc G G d 1 I BB 11

12 meq Gain Enhancement Stratey I B MQC M m is increased by the mirror ain! 1 : M C Foldin is required to establish the correct bias current direction Consider usin the quarter circuit itself to form the op amp M 1 Consider this quarter circuit Could 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 QC: OQC 1

13 meq Gain Enhancement Stratey 1 : M MQC = M M1 OEQ OQC OI BB M 1 I B Redraw to absorb I B in the quarter circuit 13

14 meq Gain Enhancement Stratey 1 : M M : 1 1 : M I B I B M 1 I B M 1 M I T 14

15 Current Mirror Op mps M : 1 1 : M C I B I B C - OUT 0 = + + IN - IN M 1 M I T ery Simple Structure! Premise: Transconductance ain increased by mirror ain M OEQ OQC 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 C meq (for += d /) meq OEQ 15

16 Current Mirror Op mps DD DD M : 1 1 : M M 5 M 3 M 4 M 6 M 1 M M 1 M I T B M 9 B1 I T M 7 M 8 Need CMFB to establish B Basic Current Mirror Op mp Can use hiher output impedance current mirrors Can use current mirror bias to eliminate CMFB but loose one output 16

17 Is this a real clever solution? 17

18 Basic Current Mirror Op mp DD M 5 M 3 M 4 M 6 M meq OEQ O6 O8 B C M 9 B1 M 1 M I T M 7 M 8 C GB M C M O O6 O8 CMFB not shown SR MI C T 18

19 Current-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 CMO? Is this a real clever solution? 19

20 Seminal Work on the OT From: N.E.C. PROCEEDINGS 0

21 Seminal Work on the OT From: 1969 N.E.C. PROCEEDINGS December

22 Oriinal OT I B I M I I 1 I I 1 I I B I OUT I 3 I 4 Q 1 Q I BC Q 3 Q 4 Current Mirror I I I B 4 I I I 3 OUT 3 I 4 I OUT M I B I

23 Oriinal OT I B I M I I 1 I MI 1 IB MI I I 1 I I B I OUT I 3 I 4 Q 1 Q I BC Q 3 Q 4 I OUT M I B I 3

24 Oriinal OT I MI 1 IB MI I I 1 I I B I OUT I 3 I 4 Q 1 Q I BC Q 3 Q 4 I OUT M I B I 4

25 Oriinal OT I MI 1 IB MI I I 1 I I B I OUT I 3 I 4 Q 1 Q I BC Q 3 Q 4 3-mirror OT I OUT M I B I 5

26 Current Mirror Op mp W/O CMFB DD M : 1 1 : M M meq Often termed an OT I T I OUT m 1 : 1 Introduced by Wheatley and Whitliner in 1969 I OUT m IN 6

27 End of ecture 10 7