EE 435. Lecture 6: Current Mirrors Signal Swing

Transcription

1 EE 435 ecture 6: Current Mirrors Signal Swing 1

2 Review from last lecture: Where we are at: Basic Op Amp Design Fundamental Amplifier Design Issues Single-Stage ow Gain Op Amps Single-Stage High Gain Op Amps Two-Stage Op Amp Other Basic Gain Enhancement Approaches

3 Review from last lecture: Single-stage low-gain differential op amp A(s) sc A O g g g O1 m1 O1 gm1 g g O3 O3 V B1 M 3 M 4 V IN M 1 M V IN GB gm1 C What are the number of degrees of freedom? (assume, fixed) V B M 5 Natural Parameters: W1 W3 W,, 5, V B1,VB Practical Parameters: V,V,V,P EB1 EB3 EB5 Need a CMFB circuit to establish V b1 Constraints: I D5 ;I D3 Net Degrees of Freedom: 4 3

4 Review from last lecture: Expressions valid for both tail-current and tail-voltage op amp V B1 M 3 M 4 V B1 M 3 M 4 V IN M 1 M V IN V IN M 1 M V IN V B M 5 So which one should be used? Common-mode input range large for tail current bias Improved rejection of common-mode signals for tail current bias Extra design degree of freedom for tail current bias Improved output signal swing for tail voltage bias (will show later) 4

5 Review from last lecture: Slew Rate Definition: The slew rate of an amplifier is the maximum rate of change that can occur at an output node V IN (t) Amplifier (t) V IN (t) V IN (t) t t (t) (t) Slope = SR - t Slope = SR + t SR is a nonlinear large-signal characteristic Input is over-driven hard (some devices in amplifier usually leave normal operating region) Magnitude of SR + and SR - usually same and called SR (else SR + and SR - must be given) 5

6 Review from last lecture: Reference Op Amp single-ended output V IN V B1 M 3 M 4 M 1 M V IN A(s) sc mixed parameters 1 g A m1 VO= g O1 +g O3 m1 GB g C g g A m1 O1 g O3 practical parameters V0 GB λ 1 1 λ P V C DD 3 1 V EB1 1 V EB1 V B M 9 IT SR C SR V P DD The Reference Op Amp (CMFB not shown) This is probably the simplest differential input op amp and is widely used Will go to more complicated structures only if better performance is required 6

7 Review from last lecture: Amplifier Structure Summary Common Source Small Signal Parameter Domain A VO g g m O g GB C m Practical Parameter Domain Common Source A VO λ 1 V EB GB P V C DD 1 V EB Small Signal Parameter Domain Reference Op Amp A VO 1 g O1 gm1 g O3 g C m1 GB SR g λc 01 Practical Parameter Domain Reference Op Amp A V0 λ 1 1 λ 3 1 V EB1 GB P V C DD 1 V EB1 SR V P DD 7

8 Single-stage low-gain differential op amp Need a CMFB circuit to establish V B1 or V B M 3 M 4 V B1 V IN M 1 M V IN CMFB Circuit V B1 V B M 5 CMFB amplifies difference between V B1 and average of two signal inputs Can apply to either V B1 or V B but not both Often apply to only fraction of transistor 8

9 Single-stage low-gain differential op amp V B1 M 3 M 4 M 3 M 4 V IN M 1 M V IN V IN M 1 M V IN CMFB Circuit V B1 V B M 5 V B M 5 Need a CMFB circuit to establish V b1 The CMFB circuit is often quite large and requires considerable design effort! Can the CMFB be removed? 9

10 Operation of Op Amp A different perspective V D D Small signal differential half-circuit V BB v d P F P F V BB V - O V + O v d V d G V G M1 V 1 1 G 1 V O I BB The signal dependent current in quarter circuit is steered to output node and drives the parallel output conductances of the quarter circuit and counterpart circuit If the signal-dependent current could be doubled, the gain would be doubled as well! A VO BW GB GM1 G G G C M1 1 G1 G C 10

11 V d V BB v d Operation of Op Amp A different perspective P F I BB V D D P F Small signal differential half-circuit G V G M1 V 1 1 G 1 V BB V - O V + O v d V O v d P F I BB V BB P V - O V + O F v d If the input impedance to the counterpart circuit is infinite and the quiescent values of the left and right drain voltages are the same, connecting the bias port of the counterpart circuit to V 0- instead of to V BB will cause the signal current in the right counterpart circuit to be equal to that in the left counterpart circuit This will approximately double the signal current steered to V o+ and thus double the voltage gain! (formal derivation on following slide) This will also eliminate the need for a CMFB circuit! 11

12 Doubling of Gain with Current Mirror connection v d P F I BB V BB P V - O V + O F v d V 3 GM3 V 3 G 3 G 4 GM4 V 4 V 4 From KC at two drain nodes Vd VOUT sc G G4 GM GM 4V Z 0 Vd VZ G1 G3 GM 3V Z GM 1 0 Eliminating V Z we obtain M 4 M1 M M 3 M 1 3 V G G G G G G G A V sc G G G G G OUT V d M 3 Assuming G M s large compared to G s and left-right symmetry, it follows that V d V Z V 1 GM1 V 1 G 1 G GM V V V d A V G M1 sc G G 4 1

13 Current Mirrors I IN I BB I OUT P P F F I I IN BB I OUT If the current I BB is small compared to I IN, and the current I IN is nearly independent of the voltage across P, then I OUT I IN Circuits with this property are called Current Mirrors If multiple copies of the right circuit are placed in parallel, the current will be scaled by the number of copies These scaled circuits are also called Current Mirrors As long as I BB <<I IN, this scaling in currents occurs even if the circuits are highly nonlinear provided the voltages across the circuits are the same! 13

14 Operation of Op Amp A different perspective v d P F V BB P V - O V + O F v d V IN M 3 M 4 I IN Basic Current Mirror M 1 M I OUT V IN I BB Consider using single n-mos transistor as quarter circuit V B M 9 Note counterpart circuits can be recognized as the basic current mirror 14

15 Current Mirrors Current mirrors are really just a current amplifier Current mirror can be used to eliminate CMFB and double gain in basic op amp Many different current mirrors exist with varying levels of performance Current mirror not necessarily from counterpart of quarter circuit but often is 15

16 Basic Current Mirror I IN I OUT μc W I V -V OX 1 IN GS1 T 1 M 1 M μc W I V -V OX OUT GS T I I W W OUT 1 IN 1 n-channel 16

17 Basic Current Mirror i IN small-signal two-port model i OUT I IN I OUT g g m1 01 gm i1 gm1 g01 g 0 M 1 M Simplified small-signal two-port model i IN i OUT m i1 g gm1 g0 m1 g n-channel i IN OR equivalently i OUT g 1 m1 W W 1 i 1 g 0 17

18 Basic Current Mirror M 1 M I IN I OUT μc W I V -V OX 1 IN GS1 T 1 μc W I V -V OX OUT GS T I I W W OUT 1 IN 1 p-channel 18

19 Current Mirrors I IN I OUT IN OUT Current Mirror (sinking) Current Mirror (sourcing) IN OUT I IN I OUT More advanced current mirrors exist Several of these are discussed in the text 19

20 Current Mirrors I IN I BB I OUT F F F F K copies of F on right I OUT Quarter circuits with high output impedance are useful for building current mirrors Replication of K copies is often simply denoted as a device sizing or scaling factor Properties of Current Mirrors of Interest: KI IN Mirror Gain Accuracy Signal Swing at Output Output Impedance (ideally infinite) More advanced current mirrors usually offer improvements in one or more of these properties 0

21 More Advanced Current Mirrors I IN I OUT I IN I OUT I IN I OUT M 3 M 4 M 3 M 4 M 4 M 1 M M 1 M M 1 M Cascode Current Mirror Wilson Current Mirror Modified Wilson Current Mirror 1

22 Current Mirrors I IN I OUT IN OUT Current Mirror (sinking) Current Mirror (sourcing) IN OUT I IN I OUT The concept of the current mirror was first introduced in about 1969 (not certain who introduced it but probably Wheatley and Wittlinger) Many of the basic current mirror circuits were introduced within a few years after the concept first appeared How many current mirror circuits are there? Have any current mirrors been introduced recently? Is there still an opportunity to contribute to the current mirror field?

23 USPTO search on Jan 1, patents with current and mirror in title since

24 USPTO search on Jan 6, patents with current and mirror in title since

25 USPTO search on Jan, patents with current and mirror in title since

26 USPTO search on Jan 1, patents with current and mirror in title since patents with current and mirror in title in 016 and 017 Averaged 1.4 patents/year from 1976 to 006 Averaged 17 patents/year in 01 and 013 Averaged 13 patents/year in 016 and 017 3

27 USPTO search on Jan 1, 018 I IN I OUT IN OUT Current Mirror (sinking) Current Mirror (sourcing) IN OUT I IN I OUT 569 patents with current and mirror in title since patents with current and mirror in title in 016 and 017 Number of patents/yearis about at the 3-decade average Is there still an opportunity to contribute to the current mirror field? 35

28 Single-stage low-gain differential op amp V B1 M 3 M 4 M 3 M 4 V IN M 1 M V IN V IN M 1 M V IN Assume left and right sides matched V B M 5 V B M 5 Can eliminate CMFB circuit if only single-ended output is needed by connecting counterpart circuits as a current mirror This will double the voltage gain and the GB as well Still uses counterpart circuits but terminated in different ways Although not symmetric, previous analysis results with specified modifications still nearly apply 45

29 Single-stage low-gain differential op amp Current-Mirror Connected Counterpart Circuit No CMFB Circuit Needed Slew Rate? When Vd large and negative, I C =-I T SR Assume left and right sides matched IT C M 3 M 4 V IN M 1 M V IN I C When Vd large and positive, I C =I T SR I T C V B I T M 5 In terms of practical parameter set SR P V C DD V V V d IN IN 46

30 Single-stage low-gain differential op amp No CMFB Circuit Needed A(s) A O sc g Current-Mirror Connected Counterpart Circuit O1 g m1 g g g O3 m1 O1 g O3 M 3 M 4 Assume left and right sides matched V IN M 1 M V IN m1 GB g C SR I C T V B I T M 5 In terms of practical design space parameters A 0 λ 1 1 λ 3 V EB1 GB P V C DD 1 V EB1 SR P V C DD 47

31 Signal Swing Consider single-input amplifier first To keep M 1 out of Triode Region 1 : > ViN -VTn V XX M To keep M 1 out of Cutoff : V > V in Tn V in M 1 To keep M out of Triode Region 3 : - > VXX VDD -VTp V XX - V Tp > VOUT 48

32 1 : Signal Swing > ViN -VTn V > V V - V > V : in Tn 3 : XX Tp OUT 1 3 V Tn V ic 49

33 Signal Swing 1 1 : > ViN -VTn : V in > VTn 3 : V XX - V Tp > VOUT 3 V Tn V ic 50

34 Signal Swing 1 V XX M 3 M 1 High Gain Region Gain = slope V in V Tn V ic Transfer Characteristics of amplifier How do the transfer characteristics relate to the signal swing? Observe signal swing boundaries are same as operating region changes for transfer characteristics 51

35 Signal Swing How do the transfer characteristics relate to the signal swing? V in V XX M M 1 V in For this circuit, high gain and large output signal swing for small V EB1 5

36 Signal Swing of Single-Stage Op Amp M 3 M 4 For high-gain amplifiers, V d is inherently very small so are only concerned about output signal swing vs V ic V M 1 M 1 V V ic V XX M 5 Generally large swings come at expense of other desirable characteristics 53

37 Signal Swing of Single-Stage Op Amp What type of signal swing is needed? V ic V ic Wide V ic and range Narrow V ic and wide range V ic V ic Narrow and wide V ic range Narrow V ic and range 54

38 Signal Swing of Single-Stage Op Amp VCC What type of signal swing is needed? VCC VSS VCC V ic VSS VCC V ic VSS Wide V ic and range Expected for catalog parts and overall I/O in many applications VSS Narrow V ic and wide range Acceptable when ViC is fixed V ic V ic Narrow and wide V ic range Acceptable when followed by high-gain stage Narrow V ic and range Acceptable when V ic fixed and followed by high-gain stage 55

39 End of ecture 6 56