Design of CMOS Analog Integrated Circuits. Basic Building Block

Transcription

1 Desin of CMOS Analo Inteated Cicuits Fanco Malobeti Basic Buildin Block F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block

2 INERTER WITH ACTIE LOAD The simplest fom of ain stae, the DC ain is ien by the slope of the cue F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/

3 Small sinal analysis: C C s + C s,o C C d + C d,o C 3 C db + C db + C d + C d,o + C L At low fequency: A out in + ds m ds Since: m µ C ox W L I D ds λi D F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 3

4 It esults: A µ I D C ox ( λ + λ ) n W L p The DC ain inceases as the squae oot of the bias cuent is deceased. This holds until the deices ente the subtheshold eion. F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 4

5 At hih fequency: Mille s theoem is applied to C The output total capacitance is C + C 3 The output esistance is /( ds + ds ) The tansfe function has one pole ω p ds C + + C ds 3 ( λ + λ ) n C p + C I 3 D The unity ain fequency is: f T ω π p A m ( 0) ID π C + C 3 π µ Cox C + C W L 3 It inceases as the squae oot of the bias cuent inceases Due to Mille s theoem the input capacitance becomes: C in C + C ( - A ) if A >> it can become sinificant to the stae diin it. F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 5

6 Example Simulate an inete with actie load ( DD 5 ) as the followin fiue with BSIM 3 Models. Find the DC ain and unity ain fequency. Obsee that the achieed ain is about 47 db; the unity ain fequency is faily ood, bein aound 500 MHz and the phase main is about 87 deees. F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 6

7 CASCODE The cascode ain stae is used to attenuate the Mille effect on node. Bias oltae such to keep M in the satuation eion B > sat, + GS sat, + Th,n + sat, "" Th,n + µ n C I ox W L + µ n C I ox W L F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 7

8 Small sinal analysis: C C s + C s,o C C d + C d,o C 4 C s + C s,o + C db + C sb C 3 C d + C d,o + C d3 + C d3,o + C db + C db3 + C L Fo low fequency, nelectin ds and ds : m in - m - ds3 0 Hence: A 0 in m ds3 A in m m The Mille effect is sinificantly educed if m m F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 8

9 At hih fequency: The cicuit has two nodes: the output and node. The capacitance at the output is C 3 The output impedance is / ds3 (nelectin the impedance at the dain of M ) The capacitance at the node is (C + C 4 ) The impedance at the node is / m The pole associated to the output node is: f p,out π τ out π ds3 C 3 The pole associated to the node is: f p, π τ π m m / ζ ( C + C4 ) + mc Whee ζ ( + ds3 / ds ) F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 9

10 Since m >> ds, f p,out is dominant. The ain-bandwidth poduct is: f T f p,dom A π C m 3 If a ood phase main is needed, it must be: C m 3 < m / ζ ( C + C4 ) + mc This conditiin can be fulfilled by inceasin C L. F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 0

11 Impedance at the dain of M: ix ix + ms x + s ds ds i x ds D i x x ds + ds + m ds ds m ds F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/

12 Impedance at the node, : x R a i x + ds ( i ) x m x s m + ds3 ds ζ m F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/

13 Tansconductance ain stae CASCODE WITH CASCODE LOAD The ain is inceased by inceasin m o out. F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 3

14 In the impoed esion the tansconductance of M is inceased by the facto ( I + I ) M 4 I M 4 M 5 B and B must keep M and M 4 out the tiode eion B > sat, + GS B < DD - sat,4 - GS3 The fiue plots the folded stuctue useful if we need to aise the oltae souce of M F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 4

15 Small sinal analysis: The output impedance is (conentional esion): out ( )( ) ds ds m m ds ds + ds4 ds4 m3 m3 ds3 ds3 [fo the impoed and folded esion ds must be eplaced with ( ds // ds5 )] The DC ain is: A m ( )( ) ds ds m m ds ds + ds 4 ds 4 m3 m3 ds3 ds3 ( ) m ds The cicuit has thee nodes: The output node The souce of M The souce of M 3 F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 5

16 The tansfe function will hae thee poles. The dominant one is the output pole f p,out π out C out f π C f 3 π C 3 3 C out, C, C 3 capacitances incident on nodes,, 3. At low fequency: out ( )( ) ds ds m m ds ds + ds4 ds4 m3 m3 ds3 ds3 + m3 ds3 ds4 ds m ds + m ds ds 3 ds4 m3 ds3 >> out, 3 F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 6

17 At hih fequency: m 3 m3 Output swin: The output swin is limited by the conditions fo which one of the tansistos of the stae is bouht out of satuation out max B + GS3 sat3 out min B + GS sat B and B must keep M, M4, and M5 out of the tiode eion. F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 7

18 Example Simulate the folded cascode amplifie, shown in the followin fiue, with DD 3.5. Use the models BSIM 3 to find the ain and the phase fom input to output and fom input to node. We obsee that the ain and the phase plots of the output show a 0 db oll-off with a ood phase main (60 deees). The low fequency ain is 77 db and the unity ain fequency is aound 80 MHz. The behaio of the ain fom the input to node is inteestin: aboe the dominant pole, it holds 4 db, just db moe than the expected alue m / m. At low fequency climbs to 34 db. F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 8

19 DIFFERENTIAL STAGE M, M in satuation with (W/L) (W/L) I I µ C W ( ) ox GS Th L µ C W ( ) ox GS Th L assume: GS GS0 + in ; GS GS0 in F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 9

20 The output aiable is the diffeential cuent: I I W ( ) I µ Cox in GS0 Th L since the bias cuent can be expessed as: I SS I + I µ C ox W L ( ) GS0 Th it esults: I in µ C ox W L I SS at small sinal: i in m with a common mode sinal: i CM m + i CMMR i CM d m CM i in i m i F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 0

21 Example eify equation I µ C (W/L) 00 µm and Iss 00 µa. in ox W L I SS. Conside an n-channel diffeential pai usin The tansconductance tansfe function is faily linea oe a wide ane of input sinal. It stats to satuate only when the input sinal appoaches the oedie oltae of the diffeential pai (75 m). F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/

22 Used as buffe o as DC-leel shifte SOURCE FOLLOWER at low fequency: ( + ) + 0 ds ds out ds out m s hence: A out in m + ds m + ds + mb F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/

23 If m >> ds + ds + mb then A at hih fequency: A ( S) C C + C out whee: C C + C + C + C + C C C + C L d d o db sb s so The output impedance is obtained by applyin a test souce x at the output node. Hence: x ( ds + ds + mb m ) x i + R out ds + ds + mb + m m The output is not symmetical. Fo n-channel input deice out max DD GS out min sat F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 3

24 Example Simulate the lae sinal behaio and deie the dc small sinal oltae ain. I B 0. ma and DD 3.3. The output oltae, pactically, follows the input shifted by GS. Howee, due to the body effect, the alue of GS is not constant; it ises fom 73 m to.3. Theefoe, the inputoutput chaacteistic is not but 0.8. The fiue shows also the dc ain: its alue anes fom 0.74 to 0.86 quite well match as theoetical esults. F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 4

25 IMPROED OUTPUT STAGES Pefomances impoed by the use of neatie feedback. x ( m + ds ) x + m4 mds 3x i R out m ( + m4ds3 ) + ds F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 5

26 Class AB push-pull: GS3 + GS4 Th,n + Th,p + I 5 µ n L 3 3 W C ox + µ p L 4 4 W C ox let: With R out 0: W L k W L W L W k 3 L 4 GS GS3; GS GS4; I I ki5; The output conductance is: + out m m With esistie load, the dop oltae acoss the output esistance detemines (oin out cuent): GS > GS3 ; GS < GS4 ; I I I out F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 6

27 Fo a ien load I --> 0; the output conductance becomes out m In eneal an output stae has the followin equialent cicuit: ( + α I + α I...) Rout Rout0 out out + It detemines hamonic distotion F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 7

28 Class AB push-pull with ain stae if it is eified the condition: m4 + m5 << ds6 F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 8

29 oltae diide Analo cicuits nomally hae only two dc oltae supplies In ode to obtain dc bias oltaes, oltae diides can be used Resistie o capacitie diides can be used, howee they ae complex o silicon aea consumin MOS in the diode confiuation can be used The tansistos ae in satuation k W L ( ) ( ) DS Th k W L + DS DS DD DS Th F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 9

30 DS α α + α DD + α DS α α + α Th α W L ; α W L It esults a oltae diision of DD plus an offset. If a sinal (usually undesied) is supeposed to DD, the small sinal equialent cicuit must be consideed. C Cs + Cdb + Csb ; C Cs At low fequency (assumin m >> ds and m >> ds ) F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 30

31 At hih fequency dd m mb + m m m + + m m mb m DD C C + C It esults an injectin of the noise fom the powe supply ( DD ) F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 3

32 Leel Shifte Essential fo NMOS cicuits, useful fo CMOS cicuits Hih-impedance leel shift Low-impedance, o battey, leel shift Hih Input Impedances: F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 3

33 L GS I + Th o + kw Th Body effect nelected Theshold oltae aiation effect ( Th +50 m) Input and output swin limitation Leel shift theshold-independent: L L ( I I ) I k W W (assumin M in satuation and nelectin λ) usually < Th F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 33

34 Low Impedances: It behaes like a oltae souce a) b) L I + kw DS Th GS + GS Th + Th + L kw I + L kw I a) Simple leel shifte b) Shunt feedback leel shifte a) out / m b) affected by twice oltae theshold aiation F. Malobeti : Desin of CMOS Analo Inteated Cicuits - Basic Buildin Block 3/ 34