EE 435 Lecture 15. Two-Stage Op Amp Design

Transcription

1 EE 435 Lecture 15 Two-Stage Op Amp Design

2 Review from Last Time Cascaded Amplifier Issues A A 0 p s p Single-stage amplifiers -- widely used in industry, little or no concern about compensation Two amplifier cascades 4β A 0TOT k 2β A 0TOT -- widely used in industry but compensation is essential! Three amplifier cascades - for ideally identical stages 3 8 βa 0 -- seldom used in industry but starting to appear! Four or more amplifier cascades - problems even larger than for three stages -- seldom used in industry! Note: Some amplifiers that are termed single-stage amplifiers in many books and papers are actually two-stage amplifiers and some require modest compensation. Some that are termed twostage amplifiers are actually three-stage amplifiers. These invariable have a very small gain on the first stage and a very large bandwidth. The nomenclature on this summary refers to the number of stages that have reasonably large gain. Results given above vary somewhat if a zero is present in the amplifier.

3 Review from Last Time Two-stage Cascade (continued) D FB (s) s 2 sp ~ kkp ~ 1 βa 1 0TOT A A 0 p s p p p ~ 2 - k j 4A kβ 1 2 1,2 0TOT k Case 1: Identical negative real-axis poles; must make discriminate 0, thus (maximally fast time-domain response w/o ringing) k 4β A 0TOT Im Im p 2 p 1 Re p 1F,p 2F Re

4 Review from Last Time Two-stage Cascade (continued) p p ~ 2 - k j 4A kβ 1 2 1,2 0TOT k A A 0 p s p Case 2: Maximally flat magnitude response; must make real and imaginary parts equal k 4A 0TOT kβ k 2 k 2β A 0TOT Im Im p 1F 45 o p 2 p 1 Re Re p 2F Small ringing in step response Factor of 2 reduction in pole spread

5 Review from Last Time Basic Two-Stage Cascade V DD V DD V d 2 P F P F V BB V OUT V BB P V d V 2 1 V 2 F V 1 V OUT I BIAS V SS Widely used structure for single-ended output Quarter circuits often different between first stage and second stage

6 Review from Last Time Basic Two-Stage Cascade V DD V DD V DD V BB1 V OUT P V BB VOUT1 P P V BB V OUT1 P VBB1 VOUT F V d 2 F F V d 2 F V SS V SS I BIAS Widely used structure for differential outputs Quarter circuits often different between first stage and second stage

7 Review from Last Time Two-stage op amp design It is essential to know where the poles of the op amp are located since there are some rather strict requirements about the relative location of the openloop poles when the op amp is used in a feedback configuration.

8 Review from Last Time Parasitic Capacitances in MOS Devices S G D CGSOL CGDOL CBS CGB when off CGC when on CBD S G D CGSOL CGDOL CBS CGB when off CGC when on CBD CWELLSSUB C GD C BD C GS C BS SUB C B-SUB C GS C GD C BD C BS Parasitic Capacitances added to Device Models C GS is often largest C BD and C BS often quite large with large drain/source area

9 Review from Last Time Pole approximation methods 1. Consider all shunt capacitors 2. Decompose these into two sets, those that create low frequency poles and those that create high frequency poles (large capacitors create low frequency poles and small capacitors create high frequency poles) {C L1, C Lk } and {C H1, C Hm } 3. To find the k low frequency poles, replace all independent voltage sources with ss shorts and all independent current sources with ss opens, all high-frequency capacitors with ss open circuits and, one at a time, select C Lh and determine the impedance facing it, say R Lh if all other low-frequency capacitors are replaced with ss short circuits. Then an approximation for the pole corresponding to C Lh is p Lh =-1/(R Lh C Lh ) 4. To find the m high-frequency poles, replace all independent voltage sources with ss shorts and all independent current sources with ss opens, replace all low-frequency capacitors with ss short circuits and, one at a time, select C Hh and determine the impedance facing it, say R Hh if all other high-frequency capacitors are replaced with ss open circuits. Then the approximation for the pole corresponding to C Hh is p Hh =-1/(R Hh C Hh )

10 Compensation of Basic Two-Stage Cascade P 1 P 2 P 1 P 2 V OUT V OUT V IN F 1 F 2 C 1 V IN F 1 F 2 C 2 Internally Compensated Output Compensated Modest variants of the compensation principle are often used Internally compensated creates the dominant pole on the internal node Output compensated created the dominant pole on the external node Output compensated often termed self-compensated Everything else is just details!!

11 Two-stage Architectural Choices Common Source Current Mirror Differential Input Single Ended Input Stage 1 Tail Voltage Tail Current Common Source Current Mirror Differential Input Single Ended Input Stage 2 Tail Voltage Output Compensated Tail Current Internally Compensated

12 Two-stage Architectural Choices Common Source Current Mirror 6 Differential Input Single Ended Input 2 Stage 1 Tail Voltage Tail Current 2 Common Source Current Mirror 6 Differential Input Single Ended Input 2 Stage 2 Tail Voltage Tail Current 2 Output Compensated Internally Compensated 2 Plus n-channel or p-channel on each stage Choices!!!

13 Two-stage Architectural Choices Common Source Current Mirror Differential Input Single Ended Input Stage 1 Tail Voltage Tail Current Common Source Current Mirror Differential Input Single Ended Input Stage 2 Tail Voltage Output Compensated Tail Current Internally Compensated Plus n-channel or p-channel on each stage Which of these 2304 choices can be used to build a good op amp? All of them!!

14 Two-stage Architectural Choices There are actually a few additional variants so the number of choices is larger Basic analysis of all is about the same and can be obtained from the quarter circuit of each stage A very small number of these are actually used Some rules can be established that provide guidance as to which structure may be most useful in a given application

15 Two-stage Architectural Choices Guidelines for Architectural Choices Tail current source usually used in first stage, tail voltage source in second stage Large gain usually used in first stage, smaller gain in second stage First and second stage usually use quarter circuits of opposite types (n-p or p-n) Input common mode input range of concern on first stage but output swing of first stage of reduced concern. Output range on second stage of concern. CMRR of first stage of concern but not of second stage Noise on first stage of concern but not of much concern on second stage Offset voltage usually dominated by that of the first stage

16 Two-stage Architectural Choices Common Source Current Mirror Differential Input Single Ended Input Stage 1 Tail Voltage Tail Current Common Source Current Mirror Differential Input Single Ended Input Stage 2 Tail Voltage Output Compensated Tail Current Internally Compensated Plus n-channel or p-channel on each stage Basic Two-Stage Op Amp

17 Two-stage Architectural Choices Common Source Current Mirror Differential Input Single Ended Input Stage 1 Tail Voltage Tail Current Common Source Current Mirror Differential Input Single Ended Input Stage 2 Tail Voltage Output Compensated Tail Current Internally Compensated Plus n-channel or p-channel on each stage -Cascade Two-Stage Op Amp

18 Two-stage Architectural Choices Common Source Current Mirror Differential Input Single Ended Input Stage 1 Tail Voltage Tail Current Common Source Current Mirror Differential Input Single Ended Input Stage 2 Tail Voltage Output Compensated Tail Current Internally Compensated Plus n-channel or p-channel on each stage -Cascade Two-Stage Op Amp

19 Basic Two-Stage Op Amp (compensated on first stage) V DD M 3 M 4 M 5 V OUT V IN M 1 M 2 V IN C C C L I T V B2 M 7 V B3 M 6 V SS o One of the most widely used op amp architectures o Essentially just a cascade of two common-source stages o Compensation Capacitor C C used to get wide pole separation o Pole on drain node of M 1 usually of little concern o Two poles in differential operation of amplifier usually dominate performance o C C can be internal (termed internally compensated) or external (termed externally compensated) o External compensation works but is usually not practical o No universally accepted strategy for designing this seemingly simple amplifier Pole spread k β A A 2 k makes C C unacceptably large for on-chip solutions

20 Basic Two-Stage Op Amp V DD M 3 M 4 M 5 V OUT V IN M 1 M 2 V IN C C C L I T V B2 M7 V B3 M 6 Pole spread βa 01 A 02 V SS makes C C unacceptably large Remember, pole spread strongly dependent upon β C C is usually an additional capacitor that is added Concept of Miller compensation will be used to reduce actual size of C C What about just making C C larger than what is needed? GB will degrade, power and area will increase What about providing additional compensation by making C L larger too? Poles will move together and degrade performance What about compensating for worst-case β=1 so β dependence can be ignored? Good solution for catalog parts so application space large but at a cost! Penalty in GB, power, and area sever if compensated for much different β than needed

21 Basic Two-Stage Op Amp V DD M 3 M 4 M 5 V OUT V IN M 1 M 2 V IN C C C L I T V B2 M 7 V B3 M 6 V SS Pole spread βa 01 A 02 makes C C unacceptably large Important to compensate just for what is needed, even a little more comes at a rather big penalty in performance, power, or area!!

22 Selected Commercial Op Amps

23 Selected Commercial Op Amps

24 Selected Commercial Op Amps

25 Selected Commercial Op Amps

26

27 Selected Commercial Op Amps Decompensated Op Amp

28 Selected Commercial Op Amps

29 Example: Sketch the circuit of a two-stage internally compensated op amp with a telescopic cascode first stage, single-ended output, tail current bias first stage, tail voltage bias second stage, p-channel inputs and n-channel inputs on the second stage.

30 Two-stage Architectural Choices Common Source Current Mirror Differential Input Single-Ended Input Differential Output Single-Ended Output Stage 1 Tail Voltage Bias Tail Current Bias Common Source Current Mirror Differential Input Single-Ended Input Differential Output Single-Ended Output Stage 2 Tail Voltage Bias Tail Current Bias Internally Compensated Output Compensated p-channel Input n-channel Input -Cascade Two-Stage Op Amp

31 Example Solution V DD V X4 V X5 V IN V IN V OUT V X3 C C

32 First Commercial Operational Amplifier K2-W Op Amp by Philbrickk,

33 Inventor of the Two-Stage Op Amp Robert Widlar Many say he started the field of analog IC design, considered a brilliant engineer Widlar began his career at Fairchild semiconductor, where he designed a couple of pioneering op amps. By 1966, the commercial success of his designs became apparent, and Widlar asked for a raise. He was turned down, and jumped ship to the fledgling National Semiconductor. At National he continued to turn out amazing designs, and was able to retire just before his 30th birthday in (from posted www site)

34 Inventor of the internally-compensated Op Amp Dave Fullagar (from posted www site) Designed the first internally-compensate op amp, the 741 Fullagar was 26 years old when this was designed (introduced?) Introduced in 1968 Largest selling integrated circuit ever Still in high-volume production even though over 40 years old Fullagar later started the linear design activities at Intersil Cofounder (catalyst) of Maxim

35 Analysis of Internally Compensated Two- Stage Op Amps P 1 P 2 V OUT V F F IN 1 2 C C C L Consider single-ended input-output (differential analysis only slightly different) Can t get everything but can get most of the small-signal results Since internally compensated, must have p 1 <<p 2

36 Analysis of Internally Compensated Two- Stage Op Amps For p 1 << p 2 A 0 A 0 A s = s s p p 1 2 p 1 p 2 ω BW p 1

37 Analysis of Internally Compensated Two- Stage Op Amps V 3 g MP1 V 3 g op1 V 4 g MP2 V 4 g op2 V OUT V IN g MF1 V 1 g of1 g MF2 V 2 V 1 C C g of2 V 2 C L

38 Analysis of Internally Compensated Two- Stage Op Amps g op1 g op2 V OUT V IN g MF1 V 1 g of1 g MF2 V 2 V 1 C C g of2 V 2 C L A V0 g gmf1 g of1 op1 g of2 gmf2 g op2 p 2 g of2 g C L op2 p 1 g of1 g C C op1 BW GB g p 1 gmf1g g of2 mf2 op2 C C

39 Analysis of Load Compensated Two-Stage Op Amps P 1 P 2 V OUT V F F IN 1 2 C 1 C C Can t get everything but can get most of the small-signal results

40 Analysis of Load Compensated Two-Stage Op Amps V 3 g MP1 V 3 g op1 V 4 g MP2 V 4 g op2 V OUT V IN g MF1 V 1 g of1 g MF2 V 2 V 1 C 1 g of2 V 2 C C

41 Analysis of Externally Compensated Two- Stage Op Amps g op1 g op2 V OUT V IN g MF1 V 1 g of1 V 1 C 1 g of2 V 2 g MF2 V 2 C C A V0 g gmf1 g of1 op1 g of2 gmf2 g op2 p 2 g of2 g C C op2 p 1 g of1 g C 1 op1 BW GB g of1 p 2 gmf1g g mf2 op1c C

42 Consider Again the Internally Compensated Two-Stage Op Amp P 1 P 2 V OUT V IN F 1 F 2 C C C L Recall approximate compensation requirements: where p2 kp 1 Thus, approximately, p2 3β A0TOT p 3β g of1 gmf1 g op1 C g C of2 gmf2 g 3β g op2 g of2 4β A k 2β 0TOT A 0TOT Since the pole ratio needs to be very large, C C gets very large! 1 g mf1 g g mf2 op2 of2 2 g C L C L op2 g of1 CC g op1

43 Miller Capacitance - Review C V 1 V 2 C 1EQ C 2EQ If V 2 = -AV 1 C 1EQ then for A large A CA C C 1 C C 1 2EQ 1 A Thus, a large effective capacitance can be created with a much smaller capacitor if a capacitor bridges two nodes with a large inverting gain!!

44 Miller Capacitance - Review C V 1 V 2 C 1EQ V 1 V 2 C 2EQ C 1EQ C 2EQ C 1EQ If V 2 = -AV 1 then for A large C 1 1 2EQ A A CA C C 1 C If A changes with frequency, C 1EQ and C 2EQ are no longer pure capacitors More useful for giving a concept than for accurate actual analysis because of frequency dependence of A

45 Miller Capacitance - Review The Basic Concept from capacitance multiplication Z IN =? -A C I X I X= Vx -(-AV X) sc = VXs C 1+A V X -A C thus VX 1 Z IN= IX s C 1+A So, if A is constant, input looks like a capacitor of value C EQ=C 1+A

46 Miller Capacitance - Review Z IN =? Cond -A C Ideal Capacitor VX 1 Z IN= IX s C 1+A A If A 0 s = s +1 p s +1+A 0 p G IN=s C 1+A sc s +1 p p Miller Capacitor ω Does not behave as a capacitor for ω > p

47 End of Lecture 15