Design of high speed op amp with different compensation technique

High speed op amp with different compensation technique7/4/201511IntroductionObjectiveLiterature SurveyFrequency CompensationCompensation Techniques Design plan ResultsLayoutConclusionReferences

Overview7/4/20152Op 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 applicationsIntroduction7/4/20153Transient Frequency

The expression for a short channel MOSFET transitionfrequency (fT) and open-loop gain (gm*ro) are given asImportant!!

Therefore from the above eqn we can see that scaling down of feature size results in higher fT7/4/20154Must use minimum length devices Larger overdrive results in faster circuits - Drawback is that the devices enter the triode region earlier For minimum power use minimum size devices - For nm CMOS minimum (drawn) W is, generally, 10 times minimum LBiasing for high speed7/4/20155Open loop gain trends in future CMOS process

The projection of open-loop voltage gain drops from CMOStransistors.

7/4/20156

7/4/20157

7/4/20158To 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.Objective7/4/20159ParametersTargetSupply voltage = 1.8V +/- 10%Vin,cm=1.2V +/-10%DC gain> 50dBUGB > 1GHzPM> 60degSettling time< 3nsCL1pFTechnology= 130nmSpecification7/4/201510Literature survey

Stability of an opampClosed loop responseBarkhausens Criteria.7/4/201511Time-Domain Response of a System Versus Position of Poles

The location of the poles of a closedLoop system is shown7/4/201512One-Pole System

(one-pole feed forward amplifier)

one pole system isUnconditionally Stable.7/4/201513Two-Pole System

The system is stable since theloop gain is less than 1 at a frequencyFor which the angle(H())=-180.When is reduced,the system becomesmore stable.

Assumption: does not dependon frequency.7/4/201514

7/4/201515

The pole locations of the classical second-order homogeneous system

The locations of the poles areIf =0, s1,2=p1,p2

7/4/201516

If 1, corresponding to an overdamped system, the two poles are real and lie in the left-halfplaneFor an underdamped system, 0 < 1, the poles form a complex conjugate pair,7/4/201517

7/4/201518Transient Response Versus PMPeaking is usually correlated with ringing in the time domain!

PM=60o, usually the optimum value.7/4/201519Frequency CompensationReason for frequency compensation:|H()| does not drop to unity when 1/gm2 ; Z=LHP

This resistor causes some cancellation of the effect of the feedforward by thefeedback.Z=1/Rc*Cc

7/4/201529Let us assume that Z>=10GBW

Max and Min Rc

7/4/201530Let us assume that Z>=10GBWFor 60deg phase margin we have Cc>0.22CLI5=SR*Cc

Design plan

For M3,M4For M1,M2For M5

For M6For M3,M4Gain 7/4/201531Another choice of Rc is to make z1 cancelP2:z1=gm6/CC(1-gm6Rz) - gm6/(CL+C1) Rc = gm6CCCC+CL+C1 7/4/20153232WKT

Optimum design for high GBW

AssumeTherefore7/4/201533Nulling resistor

DevicesValues (W/L)um(W/L)1,2 12/0.4(W/L)3,442/0.8(W/L)518/1(W/L)622/0.16(W/L)723/17/4/201534With 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 7/4/201535Small Signal Analysis

TAKING KCL AT EACHNODE7/4/201536Simplified Transfer FunctionThe 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

7/4/201537The 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

7/4/201538Analytical 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

XOXXComplex conjugateZ=UGBDominat pole7/4/201539Design planCcId1(W/L)1,2Gmc=gm1gm2P2=2UGBgm2Id2(W/L)6