ECEN474: (Analog) VLSI Circuit Design Fall 2012

Transcription

1 ECEN474: (Analo) LS Cicuit Desin Fall 0 Lectue 8: Cuent Mios Sam Palemo Analo & Mixed-Sinal Cente Texas A&M Univesity

2 Announcements & Aenda HW due Monday Readin Razavi Chapte 5 Biasin in Cs Simple Cuent Mio Tansisto Small-Sinal mpedances Simple Amplifies Othe Cuent Mio Topoloies

3 Cuent Souce Popeties Output Resistance Finite put esistance deades cuent souce accuacy and amplifie ain Othe impotant popeties: oltae headoom (compliance voltae) Accuacy Noise 3

4 How Should We Bias Ou Cicuits? Resistive Biasin Assumin satuation D µ nc µ nc ox W L R ox G W L RG R ( ) G G is sensitive to Supply (dd) Tn dd Pocess ( Tn and µ n C ox W/L) Tempeatue ( Tn and µ n ) Tn 4

5 C Biasin n C desin we often assume that we have one pecise cuent souce and we copy its value to ou cicuits 5

6 Simple Cuent Mio That copy cicuit is a cuent mio Simple Cuent Mio 6 ( ) Tn ox n REF G Tn G ox n REF D L W C L W C? µ µ G What is f G is applied to anothe tansisto Tn Tn ox n REF ox n L W C L W C µ µ REF L W L W

7 deal Cuent Mio Example ma ma 3 0.5mA 4.5mA This bias scheme educes sensitivity to pocess, voltae, and tempeatue vaiations 7

8 CS Amplifie w/ Cuent Souce 8 What is? Need to insue that M3 emains in satuation ( ) Tn ox n D G G G Tn ov G s L W C dd R R R µ

9 TAMU-ELEN Cuent Mios: Accuacy limitations Jose Silva-Matinez n eneal (W/L) N(W/L), most pobably T T, then D µ C n OX W L ( ) ( λ ) GS T DS GS M M D µ nc µ nc OX OX W L W L ( ) ( λ ) GS ( ) ( λ ) GS T T DS DS D K K P P ( GS T ) ( λds ) ( ) ( λ ) GS T DS N D λ Eo λ L DS λds Lon devices educe the eo; make DSDS Good solution > use cascode stuctues Eo K P K P Eos can be educed (but not eliminated) by usin eplicas of the main device and ood lay! Eo T T Effective mobility and theshold voltaes ae sensitive to DS.and dsat - 9 -

10 TAMU-ELEN DC Cuent Mios: Second-Ode Effects Jose Silva-Matinez Afte ood lay: Toleances in N ae in the ane of 0.5- %. Usually mismatches ae invesely popotional to ate aea! D D µ µ n n ( ) ( λ ) GS T DS ( ) ( ) N λ GS T DS GS M Eo M T Eo is minimized by usin eplicas of the basic device nta-die T mismatches ae invesely popotional to ate aea! T Mobility deadation Eo µ µ T T0 µ µ 0 θ -3µm s Lε ds cit W

11 Small-Sinal mpedance: Simple Cuent Souce o

12 Small-Sinal mpedance: Diode Load m o m

13 Small-Sinal mpedance: Lookin nto Souce 3 ( ) ( ) m o mb m o o mb m o o o mb m o v v v i

14 Small-Sinal mpedance: Lookin nto Souce w/ Dain Resisto m mb o R D o 4

15 TAMU-ELEN Jose Silva-Matinez Small sinal analysis: Common-souce amplifie : M Small sinal equivalent v v s m v s 0 0 v f OUT > DSAT, OUT <DD- SAT GS v s M v Av 0 m 0 m, 0, and 0 ae function of Q Q Opeatin point GS v TH v GSQ - 5 -

16 TAMU-ELEN Jose Silva-Matinez Small sinal analysis: Common-dain (souce followe) amplifie Small sinal equivalent cicuit GS M A v in M v v in v in v s m v s mb v bs v How this is done? Why? v s m v in / m / mb v v v in m m mb

17 TAMU-ELEN Jose Silva-Matinez Small sinal analysis: Common-ate amplifie (cascode) : M Small sinal equivalent cicuit v M v v in v s - m v s mb v bs 0 0 v N v in m v in 0 0 v v in m 0 mb 0 0 v in mb v in mpedance seen at in and? Ae they elevant? - 7 -

18 TAMU-ELEN Pecise Cuent Mios: Cascode stuctue Eo λ DS λds Jose Silva-Matinez??? GS GS M3 M M4 M GS M AND M ARE ACTNG AS CURRENT MRRORS FUNDAMENTAL PRNCPLE: Eo can be educed if and only λ λ and DS DS Tansistos M and M ae used as cuent mios Tansistos M3 and M4 ae used to have DS DS. LL and DD>λλ f MM then µµ, and TT - 8 -

19 TAMU-ELEN Output impedance: Cascode stuctue Jose Silva-Matinez (nelectin mb4 ) s m4 v s4 S4 0 v s4 v 0 i d 04 i D D 0 i i d d OUTPUT MPEDANCE Z v i ( v v ) 0 0 v s4 s4 0 d 04 Z 0 04 m4 m4 v s Notice that most of the AC cuent e-ciculate within the cascode device and only i d is extacted fom v 0!! Compae m4 with 0! - 9 -

20 TAMU-ELEN Jose Silva-Matinez Compaison of cuent souces: put impedance and headoom GS M 0 M v 0 GS GS m4 0 M3 M 04 M4 M DS 4 GS ~ m ~ ! 0 > DSAT 0 > GS DSAT4 T DSAT DSAT4 ~ m ~ -.5! - 0 -

21 TAMU-ELEN Jose Silva-Matinez Double Cascode Stuctue: Advantaes and dawbacks! G4 M4 DS 4 Small sinal put esistance: m4 ( ) ( ) eq3 04 m4 m G3 0 > DS DSAT 3 DSAT 4 M3 DS 3 Output esistance is inceased GS M M DS oltae swin is educed Paasitic poles could be an issue Usually this section is moe complex to ensue DS is simila in both tansistos M and M How G3 and G4 can be eneated???? - -

22 TAMU-ELEN oltae efeences (biasin cascode stuctues) Jose Silva-Matinez G3 Lets conside the case: DS DSAT M3 DS3 G3 must be GS3 DSAT T3 DSAT3 DSAT GS M M DS GSR TR µ n C OX L W R R D MR DSATR MR GS G3 M3 M M DS3 X DS nceasin L/W by 4, DSAT inceases by Accodin to (W/L), the ate dimensions fo MR (W/L) R must be desined Poblem: TR T3 due to body effect Patial solution: (W/L) > 4(W/L) R 9(W/L) R - -

23 TAMU-ELEN mpoved (self eulated) cuent souce Jose Silva-Matinez -A s4 M4 i D M3 DS4 m4 (A )v s4 04 GS M M DS S4 i D D 0 m4-3 - ( A ) 004 Simila to double cascode Key issue: Please undestand the concept!!

24 TAMU-ELEN Othe cuent souces Jose Silva-Matinez M3 M4 Wilson Cuent Souce m4 ( A )(" ") eq 04 M M " eq " / m Output esistance simila to cascode M, M3 in tiode eion M, M4 ae satuated M M M4 M3??,min??? - 4 -

25 TAMU-ELEN Othe cuent souces Jose Silva-Matinez M M M4 M3 R M, M3 in tiode eion M, M4 ae satuated i D ds 3 µ C OX µ C L OX W eff W [ 0.5 ] GS L eff [ ] GS 3 T T DS 3 DS DS m 4 04 ( ) ds 3 0 > DS 3 DSAT 4 R Satuated Tiode Tiode Satuated Tiode Tiode m 4 04 ( ) dsi Tiode Tiode - 5 -

26 Low-oltae Cascode Cuent Mio M and M4 should be sized such that GS GS4 M and M3 biased nea ede of satuation DS DS3 DSAT b GS ( GS - T ) GS4 ( GS3 - T3 ) mpoved b eneation cicuit M5 sized such that GS5 GS Some body effect eo hee Size M6 and Rb such that DS6 GS6 -R b GS - T DD DD bias in b M B M M 4 A B M M 3 Compliance oltae DSAT 3 DSAT 4 [Razavi] 6

27 Next Time Sinle-Stae Amplifies 7