EE 435. Lecture 5 Spring Fully Differential Single-Stage Amplifier Design

Transcription

1 EE 435 ecture 5 Sprin 06 ully Differential Sinle-Stae mplifier Desin Common-mode operation Desin of basic differential op amp Slew Rate The Reference Op mp

2 Review from last lecture: Determination of op amp characteristics from quarter circuit characteristics -- The differential ain -- Small sinal Quarter Circuit Small sinal differential amplifier DD I XX IN OUT OUT d BB d OUT OQC BW GB SS G G C G G M M Odif f G GB C GM G G M I BIS G G BW C or SS Note: actor of 4 reduction of ain

3 Review from last lecture: Sinle-stae low-ain differential op amp -- The differential ain -- DD B M 3 M 4 OUT SS IN M M IN Quarter Circuit Sinle-Ended Output : Differential Input Gain (s) sc m O O m O m GB C B M 5 Need a CMB circuit to establish b 3

4 Review from last lecture: Common-Mode and Differential-Mode nalysis Consider any output voltae for any linear circuit with two inputs inear Circuit B OUT c = + OUT= + OUT = c c + dd - d= - OUT = c + d Implication: Can solve a linear two-input circuit by applyin superposition with and as inputs or by applyin c and d as inputs Implication: In a circuit with = -, C =0 we obtain = OUT d d 4

5 Review from last lecture: Common-Mode and Differential-Mode nalysis Extension to differential outputs and symmetric circuits OUT inear Circuit OUT E E B B Differential Output Symmetric Circuit with Symmetric Differential Output Theorem: The differential output for any linear network can be expressed equivalently as OUT= + or as OUT = c c + dd and superposition can be applied to either and to obtain and or to c and d to obtain c and d Theorem: The symmetric differential output voltae for any symmetric linear network excited at symmetric nodes can be expressed as OUT = d d where d is the differential voltae ain and the voltae d = - 5

6 Common-Mode and Differential-Mode nalysis Consider any output voltae for any linear circuit with two inputs inear Circuit OUT B OUT= + OUT = c c + dd inear Circuit B OUT d inear Circuit B OUT d inear Circuit B OUT c inear Circuit B OUT c Sinle-Ended Superposition Difference-Mode/Common-Mode Superposition 6

7 Common-Mode and Differential-Mode nalysis Consider an output voltae for any linear circuit with two inputs inear Circuit OUT B OUT = c c + dd Difference-Mode/Common-Mode Superposition is almost exclusively used for characterizin mplifiers that are desined to have a lare differential ain and a small common-mode ain nalysis to this point have focused only on the circuit on the left d inear Circuit B OUT inear Circuit c B d OUT c 7

8 erformance with Common-Mode Input DD DD OUTC c BB c OUTC OUTC c BB c OUTC I BIS SS Sinle-Ended Outputs Tail-Current Bias DD Sinle-Ended Outputs Tail-oltae Bias DD OUT c BB ODC c OUT OUT c BB ODC c OUT I BIS SS Differential Output Tail Current Bias Differential Output Tail oltae Bias 8

9 erformance with Common-Mode Input Consider tail-current bias amplifier DD DD OUTC c BB c OUTC OUTC BB I BIS c DD OUTC BB OUTC I BIS / c c Common-Mode Half-Circuit I BIS I SYM I BIS 9

10 erformance with Common-Mode Input Consider tail-current bias amplifier DD G BB OUTC OUTC C G M G c I BIS / X OUTC sc+g +G +G M = G x C = + X XG - G M = OUTC G Solvin, we obtain Common-Mode Half-Circuit OUTC =0 thus C =0 (Note: Have assumed an ideal tail current source in this analysis C will be small but may not vanish if tail current source is not ideal) 0

11 erformance with Common-Mode Input Consider tail-voltae bias amplifier DD DD OUTC c BB c OUTC OUTC BB SS c DD SS OUTC c BB c OUTC Common-Mode Half-Circuit SS I SYM SS

12 erformance with Common-Mode Input Consider tail-voltae bias amplifier DD G OUTC BB C G M G OUTC c SS OUTC M = C sc+g +G +G = 0 Common-Mode Half-Circuit Solvin, we obtain OUTC -G = M C= C sc+g +G This circuit has a rather lare common-mode ain and will not reject common-mode sinals Not a very ood differential amplifier

13 EE 435 ecture 5 Sprin 06 ully Differential Sinle-Stae mplifier Desin Common-mode operation Desin of basic differential op amp Slew Rate The Reference Op mp 3

14 Recall Sinle-stae low-ain differential op amp DD B M 3 M 4 OUT SS IN M M IN Quarter Circuit Sinle-Ended Output : Differential Input Gain (s) sc m O O m O m GB C B M 5 Need a CMB circuit to establish b 4

15 Desin of Basic Sinle-stae low-ain differential op amp (s) sc O O m O m DD B M 3 M 4 OUT IN M M IN GB m C What are the number of derees of freedom? (assume DD, fixed, Symmetry) B M 5 Natural arameters: W W3 W,, 5, B,B 3 5 ractical arameters:,,, EB EB3 EB5 Need a CMB circuit to establish b Constraints: I D5 ;I D3 Net Derees of reedom: 4 Will now express performance characteristics in terms of ractical arameters 5

16 Desin of Basic Sinle-stae low-ain differential op amp DD B M 3 M 4 OUT SS IN M M IN Quarter Circuit Sinle-Ended Output : Differential Input Gain m (s) sc O m O 0 O m GB C λ λ 3 B EB M 5 Need a CMB circuit to establish b GB C DD Have 4 derees of freedom but only two practical variables impact 0 and GB so still have DO after meet 0 and GB requirements 6 EB

17 Sinle-stae low-ain differential I/O op amp Quarter Circuit OD O O DD B M 3 M 4 OUT OUTD OUT IN M M IN (s) sc m O O m GB C SS Differential Output : Differential Input Gain m O 0 λ λ 3 EB GB C Have 4 derees of freedom but only two practical variables impact 0 and GB so still have DO after meet 0 and GB requirements Need a CMB circuit to establish B or B B DD M 5 EB 7

18 Expressions valid for both tail-current and tail-voltae op amp Recall: OUT DD B M 3 M 4 IN M M IN OUT OUT DD B M 3 M 4 OUT IN M M IN W W, 3, B 3 So which one should be used? Common-mode input rane lare for tail current bias Improved rejection of common-mode sinals for tail current bias Two extra desin deree of freedom for tail current bias Improved output sinal swin for tail voltae bias (will show later) B M 5 W W3 W,, 5, B,B

19 Slew Rate Definition: The slew rate of an amplifier is the maximum rate of chane that can occur at an output node IN (t) mplifier OUT (t) IN (t) IN (t) t t OUT (t) OUT (t) Slope = SR - t Slope = SR + SR is a nonlinear lare-sinal characteristic Input is over-driven (some devices in amplifier usually leave normal operatin reion) Hard input overdrive depicted in this fiure Manitude of SR + and SR - usually same and called SR (else SR + and SR - must be iven) t 0

20 Slew Rate DD B M 3 M 4 OUT OUT IN M M IN B M 5 With step input on IN+, all tail current (I T ) will o to M thus turnin off M thus current throuh M 4 which is ½ of I T will o to load capacitor The I- characteristics of any capacitor is d I=C dt Substitutin I=I T /, = OUT+ and C= obtain a voltae ramp at the output thus + + dout IT SR dt C C DD ractical parameter domain

21 Slew Rate DD B M 3 M 4 OUT OUT IN M M IN B M 5 It can be similarly shown that puttin a neative step on the input steer all current to M thus the current to the capacitor will be I T minus the current from M which is still I T /. This will cause a neative ramp voltae on OUT+ of value + - dout IT SR dt C C DD Since the manitude of SR + and SR - are the same, obtain a sinle SR for the amplifier of value SR C DD

22 IT SR C Sinle-stae low-ain differential op amp Consider sinle-ended output performance : Will term this the reference op amp Will make performance comparisons of other op amps relative to this (s) sc mixed parameters m O= O + C m GB m O 0 GB practical parameters λ SR λ C DD DD 3 EB EB IN OUT DD B M 3 M 4 M M B The Reference Op mp (CMB not shown) IN IN M 9 OUT OUT 3 IN

23 Reference Op mp sinle-ended output IN OUT DD B M 3 M 4 M M OUT IN (s) sc mixed parameters m O= O + m GB C m O practical parameters 0 GB λ λ C DD 3 EB EB B M 9 IT SR C SR DD The Reference Op mp (CMB not shown) This is probably the simplest differential input op amp and is widely used Will o to more complicated structures only if better performance is required 4

24 mplifier Structure Summary Common Source Small Sinal arameter Domain O m O GB C m ractical arameter Domain Common Source O λ EB GB C DD EB Small Sinal arameter Domain Reference Op mp O O m C m GB SR λc 0 ractical arameter Domain Reference Op mp 0 λ λ 3 EB GB C DD EB SR DD 5

25 Reference Op mp sinle-ended output What basic type of amplifier is this op amp? DD B M 3 M 4 OUT OUT IN M M IN oltae Transconductance I T B M 5 Transresistance Current (s) sc m O 6

26 Reference Op mp sinle-ended output What basic type of amplifier is this op amp? Does it really matter? Transconductance DD B M 3 M 4 OUT OUT IN M M IN oltae Transconductance I T B M 5 (s) sc m O Transresistance Current 7

27 End of ecture 5 8