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Transcript of Design of High Speed Op Amp With Different
04/27/2023 1
KLE Society’s B.V. Bhoomaraddi College of Engineering and
Technology,Hubli
High speed op amp with different compensation techniques
Under the guidance of Dr. R B Shettar
Mentor:Rakesh Sawant
By, Mr. Abhishek C Math
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About Internship Introduction Objective Literature Survey Frequency Compensation Compensation Techniques Design plan Results Layout Conclusion References
Overview
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Company : SmartPlay, founded in 2008, currently has design centres at
San Jose, San Diego and Austin in the US, and at Bangalore and Greater Noida in India.
CEO: Pradeep Vajram. SmartPlay Technologies is an established design services
company with expertise in digital, analog, wireless software and system design.
It is ranked as second largest VLSI Design service company in India by Cyber Media Research Ltd.
Number of employees: +1200
About Internship
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ServicesSmartPlay offers a comprehensive range of services across multiple industry segments:
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At present, they catered to close to 30 clients in the US, India Taiwan, Japan, Korea and China.
The main Clients : Qualcomm Broadcom Texas Instrument Intel NXP Semiconductor AMD
Clients
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Where I was
Mixed Analog Mode Design
Digital Domain
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Task
Design of high speed two stage op amp with different types of
compensation circuits .
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Op Amps are the most versatile and integral part of many analog and mixed signal system.
They are employed from dc bias application to high speed amplifiers and filters
General purpose op amps can be used as summer, buffer, integrator, differentiator, comparators and many other applications
Introduction
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Transient FrequencyThe expression for a short channel MOSFET transitionfrequency (fT) and open-loop gain (gm*ro) are given as
Important!!
Therefore from the above eqn we can see that scaling down of feature size results in higher fT
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Must use minimum length devices Larger overdrive results in faster circuits For minimum power use minimum size devices - For nm CMOS minimum (drawn) W is, generally, 10 times
minimum L
Biasing for high speed
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Open loop gain trends in future CMOS process
The projection of open-loop voltage gain drops from CMOStransistors.
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Transistor transition frequency (fT) trends in future CMOS processes
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To design the high speed two stage op amp with different types of compensation circuits like
1. with nulling resistor 2. with voltage follower 3. with current buffer
and the optimized layout design.
Objective
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Parameters TargetSupply voltage = 1.8V +/- 10%Vin,cm =1.2V +/-10%DC gain > 50dBUGB > 1GHzPM > 60degSettling time < 3nsCL = 1pFTechnology = 130nm
Specification
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Literature surveyStability of an opamp
Closed loop response
“Barkhausen’s Criteria”.
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Time-Domain Response of a System Versus Position of Poles
The location of the poles of a closedLoop system is shown
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One-Pole System(one-pole feed forward amplifier)
one pole system isUnconditionally Stable.
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Two-Pole System
The system is stable since theloop gain is less than 1 at a frequencyFor which the angle(βH(ω))=-180.
When β is reduced,the system becomesmore stable.
Assumption:β does not dependon frequency.
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The pole locations of the classical second-order homogeneous system
The locations of the poles are
If β=0, s1,2=ωp1,ωp2
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If ζ ≥ 1, corresponding to an overdamped system, the two poles are real and lie in the left-halfplane
For an underdamped system, 0 ≤ ζ < 1, the poles form a complex conjugate pair,
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Transient Response Versus PM Peaking is usually correlated with ringing in
the time domain!
PM=60o, usually the optimum value.
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Frequency Compensation Reason for frequency compensation:
◦ |βH(ω)| does not drop to unity when <βH(ω) reaches -180o.
◦ Possible Solution:
(Push PX Out) (push GX in)
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Miller Theorem
The method that “transforms” a floating capacitor to two grounded capacitors, thereby allowing association of one pole with each node
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Two stage OTAWithout Rz
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Small signal analysis
The small signal transfer function for the two stage miller compensation
Compensation capacitor (Cc) between the output of the gain stages causes pole-splitting and achieves dominant pole compensation.
The first pole is located at
The second pole is located at
An RHP zero exists at
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Pole splitting with CcPole splitting with gm2
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What is the origin of the positive zero??
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Origin of positive zero
This is the result of the feedforward current which adds extra phase margin -90deg
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The different types of compensation circuit to remove zero is With nulling resistor
With current buffer
With voltage buffer
Types of compensation techniques
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With the nulling resistor
If Rc=1/gm2 ; Z=infinity
If Rc>1/gm2 ; Z=LHP
This resistor causes some cancellation of the effect of the feedforward by thefeedback.
Z=1/Rc*Cc
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Let us assume that Z>=10GBW
Max and Min Rc
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Let us assume that Z>=10GBWFor 60deg phase margin we have Cc>0.22CL I5=SR*Cc
Design plan
For M3,M4
For M1,M2
For M5
For M6
For M3,M4
Gain
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Another choice of Rc is to make z1 cancelP2:
z1=gm6/CC(1-gm6Rz) ≈ - gm6/(CL+C1)
Rc = gm6CC
CC+CL+C1
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WKT
Optimum design for high GBW
AssumeTherefore
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Nulling resistor
Devices Values (W/L)um
(W/L)1,2 12/0.4(W/L)3,4 42/0.8(W/L)5 18/1(W/L)6 22/0.16(W/L)7 23/1
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With current buffer
The compensation current is indirectly feedback from low impedance nodeThe RHP pole zero can be eliminated as the feedforward current is blocked
by the common gate amplifierNode V1 is now not loaded by the compensation capacitor (as previously)
and thus results in a much faster second stage and increased unity gain frequency
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Small Signal AnalysisCc
gm1Vd
R1 C1
V1 VA
gmcVA
roc
1/gmc RA CA gm5V1 R2CL
Vout
TAKING KCL AT EACHNODE
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Simplified Transfer Function The transfer function can be
simplified and approximated as:-
The coefficients can be evaluated as
Evaluating the poles and zeros Assuming the pole |p1| >> |p2|, |p3|
The denominator can now be approximated
Complex Poles
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The third order transfer function as 3 poles and 1 zeroDominant Pole location
Non-dominant poles are complex conjugate Condition For complex Poles
LHP Zero Location
bserving the Pole/Zero Locations
Remains at the same location
Improves Phase Margin
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Analytical Results SummaryPole / Zero Location
Complex Poles Condition
Quick Facts Complex P2,P3 moved to much
higher frequency
Z=UGB
LHP zero improves the phase margin near UGB
Much faster op-amp with lower power and CC
XO
X
X
Complex conjugate Z=UGB
Dominat pole
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Design plan
Cc
Id1
(W/L)1,2
gmc=gm1
gm1 P2=2UGB
gm2
Id2
(W/L)6
<-Noise
<- UGB
<-SR=Id1/Cc
<-Assume Z=UGBId1=Idc
<-For 60deg PM
<-gm2=4gm1
<-SR=Id2/CL
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Current buffer
Devices Values (W/L)um
(W/L)1,2 72/0.8(W/L)3,4 48/0.8(W/L)5 12/1(W/L)6 84/0.15(W/L)7 36/1(W/L)8 (CG) 12/0.6(W/L)9 12/1
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Voltage buffer
Devices Values (W/L)um
(W/L)1,2 16/0.8(W/L)3,4 38/0.8(W/L)5 16/1(W/L)6 34/0.15(W/L)7 26/1(W/L)8 (VB) 14/0.6(W/L)9 6/1
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With current buffer
Results
DC GAIN
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Settling time
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Slew Rate
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Current buffer
Parameter Simulated values
UGB 1GHzDC gain 59.8dBPM 61.12degCc 0.4pFSettling time 3.4ns(2%)Slew Rate 707V/usPdiss 2.66mW
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With nulling resistor
Parameter Nulling resistor
UGB 235MHzDC gain 61dBPM 60.15degCc 1.2pFPdiss 1.94mW
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With voltage follower
Parameter Voltage follower
UGB 478MHzDC gain 54dBPM 60.9degCc 0.7pFPdiss 2.22mW
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Parameter Nulling resistor
Voltage follower
Current buffer
UGB 235MHz 478.84MHz 1GHzDC gain 61dB 54.4dB 59.8dBPM 60.15deg 60.9deg 61.12degCc 1.2pF 700fF 400fFPdiss 1.94mW 2.22mW 2.66mW
Comparison
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Layout of current buffer
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The three compensated OTA is compared with gain, UGB, Power dissipation,PM.
We can observe that Cc of current buffer is reduced to 0.4pF when compared to Cc of nulling resistor of 1.2pF.
Current buffer compensated OTA there is improvement in the gain, UGB, PM and area requirement is also less compared to nulling resistor compensated OTA.
Conclusion
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[1] X. H. Fan, C. Mishra, and E. Sanchez-Sinencio, "Single miller capacitor frequency compensation technique for low-power multistage ampliers” IEEE Journal of Solid-State Circuits, vol. 40, no. 3, pp. 584-592, 2005.
[2] P. R. Gray, and R. G. Meyer,, "Analysis and design of analog integrated circuits,"' ,3rd ed., New York: Wiley, 1993.
[3] G. Palmisano, and G. Palumbo, "A compensation strategy for two-stage CMOS opamps based on current buer" IEEE Transactions on Circuits and Systems I-Fundamental Theory and Applications, vol. 44, no. 3, pp. 257-262, Mar, 1997.
[4] B. K. Ahuja, An Improved Frequency Compensation Technique for CMOS Operational-Ampliers" IEEE Journal of Solid-State Circuits, vol. 18, no. 6, pp. 629-633, 1983
[5] K. N. Leung, and P. K. T. Mok,"Analysis of multistage amplier frequency compensation" IEEE Transactions on Circuits and Systems I-Fundamental Theory and Applications, vol. 48, no. 9, pp. 1041-1056, Sep, 2001.
[6] H. Lee, and P. K. T. Mok, Active-feedback frequency-compensation technique for low-power multistage ampliers" IEEE Journal of Solid-State Circuits, vol. 38, no. 3, pp. 511-520, 2003.
[7] D.A.Johns and K.Martin, "Analog Integrated Circuit Design", New York: John Wiley and Sond,Inc.,1997.
[8] A. Pugliese, F. A. Amoroso, G. Cappuccino et al., "Design approach for fastsettling two-stage ampliers employing current-buer Miller compensation" Analog Integrated Circuits and Signal Processing, vol. 59, no. 2, pp. 151-159, 2009.
References
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[9] H. Mahattanakul, and J. Chutichatuporn, "Design procedure for two- stage CMOS opamp with exible noise-power balancing scheme" IEEE Transactions on Circuits and Systems I-Regular Papers, vol. 52, no. 8, pp. 1508-1514, 2005.
[10] R. J. Reay, and G. T. A. Kovacs,, "An Unconditionally Stable 2-Stage CMOS Amplier" IEEE Journal of Solid-State Circuits, vol. 30, no. 5, pp. 591-594, 1995.
[11] A. Pugliese, F. A. Amoroso, G. Cappuccino et al., "Design approach for fastsettling two-stage ampliers employing current-buer Miller compensation" Analog Integrated Circuits and Signal Processing, vol. 59, no.2, pp. 151-159, 2009.
[12] A. Pugliese, F. Amoroso, G. Cappuccino et al., "Settling time optimisation for two-stage CMOS ampliers with current-buer Miller compensation" Electronics Letters, vol. 43, no. 23, pp. 1257-1258, 2007.
[13] M. Loikkanen, and J. Kostamovaara, "Improving capacitive drive capability of two-stage op amps with current buer" Proceedings of the 2005 European Conference on Circuit Theory and Design, Vol 1, pp. 99-102,2005.
[14] Mahattanakul, J., "Design Procedure for Two-Stage CMOS Operational Ampliers Employing Current Buer" IEEE Transaction on Circuits and Systems II- Express Briefs, vol. 52, no. 11, Nov 2005.
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[15] Saxena, V. and Baker, R. J., "Indirect Compensation Technique for Low Voltage Op-Amps" Proceedings of the 3rd Annual Austin Conference on Integrated Systems and Circuits (ACISC), May 7-9, 2008.[16] Behzad Razavi " Design of analog cmos integrated circuits" McGrawHill, 2001.
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[18] Baker, R.J., "CMOS: Circuit Design, Layout, and Simulation" 2nd Ed.,Wiley Inter science, 2005.
[19] Dan Clein, "CMOS IC LAYOUT Concepts, Methodologies, and Tool”2nd Ed., Wiley Inter science, 2005.
[20] Alan Hastings, "The Art of Analog Layout (Sencond Edition)" 2007.IEEE Journal of Solid-State Circuits, vol. SC-14, Issue 6, Dec.1979, pp.1111-1114. 1996.
[21] Pelgrom, M. J. M., Duinmaiger, A. C. J., andWelbers, A. P. G, "Matching Properties of MOS Transistors" 2007. IEEE J. Solid-State Circuits,Vol. SC-24, Oct. 1989, pp. 1433-1439.
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THANK YOU