Solving Op Amp Stability Issues

Solving Op Amp Stability Issues

(For Voltage Feedback Op Amps)Tim Green & Collin WellsPrecision Analog Linear Applications

1

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OverviewMain Presentation Focus:1) Op Amp Stability Basics2) Stability Analysis – Method 1 : Loaded Aol & 1/b Technique

A) Riso Compensation Technique for Output Capacitive Loads3) Stability Analysis – Method 2 : Aol & 1/b Technique

A) CF Compensation Technique for Input Capacitance 4) Stability Tricks and Rules of Thumb

Appendix:5) Additional Useful Tools for your Analog Stability Toolbox

A) Op Amp Output ImpedanceB) Pole and Zero: Magnitude and Phase on Bode PlotsC) Dual Feedback Paths and 1/bD) Non-Loop Stability Problems

2) Nine different ways to stabilize op amps with capacitive loadsA) Definition by example using TINA-TI simulations

The CulpritsOutput Capacitive Loads!

Input Capacitance and Large Value ResistorsTransimpedance Amplifiers!

Reference Buffers! Cable/Shield Drive! MOSFET Gate Drive!

Large Value Resistors or Low-Power Circuits!

-

+

IOP2

R3 499kR4 499k

+

VG2

Cin 25p Vout

-

+

OPA

Cin 1u

C1 1u

C2 1u

VIN 5Vin

Temp

GND

Vout

Trim

U1 REF5025

C3 10u

ADC_VREF

C4 100n

-

+

OPA

RL 250

Rf 20kRg 1k

+

Vin

-

+

OPA

C_Cable 10nVout

VREF 2.5

VREF

Shielded Cable

-

+

OPA

VRef 2.5

R1 20k

R2 20k

Vin 10

VRegQ1

RL 200

Vo

Rf 1M

Rd 4.99G Cd 10p-

+

OPA

Id

Photodiode Model

Transient Suppression!V+

-

+

OPA

Rf 49kRg 4.99M

Cd 200p Vout

+Vin

D1

D2TVS

5

Just Plain Trouble!Inverting Input Filter??

Output Filter??

-

+

OPA

VoutV1 5R1 10k

R2 49k C1 10u C5 100n

-

+

OPA

Rf 100kRg 10k

Cin 1u

Vout

+

Vin

Oscillator

OscillatorT

Time (s)1.95m 2.23m 2.50m

VG1

0.00

10.00m

Vfb

-37.08m

62.12m

Vo

-1.00

1.16

T

Time (s)1.95m 2.23m 2.50m

VG1

0.00

10.00m

Vfb

-37.08m

62.12m

Vo

-1.00

1.16

6

Vcc

Vcc

VOUT

+

-

+3

4

52

1

U1 OPA333

Vcc 5V

R2 49kOhm

R1 49kOhm

C1

VIN

CLoad 1uF

100nF

+2.5V

+2.5V

T

Time (s)0 500u 1m 2m 2m

VOUT

1.5

2.0

2.5

3.0

But it worked fine

in the lab!Transient on:+Input or –InputVcc or VeeOutput

7

Check ALL Op Amp Circuits for Stability regardless of their closed loop signal frequency of operation!

But I’m only using it at DC!

Recognize Amplifier Stability Issues on the Bench• Required Tools:

– Oscilloscope– Signal Generator

• Other Useful Tools:– Gain / Phase Analyzer– Network / Spectrum Analyzer

8

Recognize Amplifier Stability Issues• Oscilloscope - Transient Domain Analysis:

– Oscillations or Ringing– Overshoots– Unstable DC Voltages– High Distortion

T

Time (s)1.75m 2.25m 2.75m

Vol

tage

(V)

0.00

18.53m

T

Time (s)1.75m 50.88m 100.00m

Out

put

-14.83

0.00

15.00T

Time (s)1.75m 2.25m 2.75m

Vol

tage

(V)

0.00

21.88m

9

Recognize Amplifier Stability Issues• Gain / Phase Analyzer - Frequency Domain: - Peaking, Unexpected Gains, Rapid Phase Shifts

T

Gai

n (d

B)

-60.00

-40.00

-20.00

0.00

20.00

40.00

Frequency (Hz)1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M

Pha

se [d

eg]

-360.00

-180.00

0.00

10

Quick Op-Amp Theory Bode Plot ReviewBasic Stability Tools

11

Poles and Bode Plots

+90

-90

+45

+-45

10 100 1k 10k 100k 1M 10M

Frequency (Hz)0

(d

egre

es)

-45o @ fP

-45o/Decade

-90o

0o

0

20

40

60

80

100

10M1M100k10k1k100101

Frequency (Hz)

A (d

B)

-20dB/Decade-6dB/Octave

fPG

0.707G = -3dB

Actual Function

Straight-Line Approximation

R

CVIN

VOUT

A = VOUT/VIN

Single Pole Circuit Equivalent

X100,000

Pole Location = fP

Magnitude = -20dB/Decade Slope Slope begins at fP and continues down

as frequency increases Actual Function = -3dB down @ fP

Phase = -45°/Decade Slope through fP

Decade Above fP Phase = -90° (-84.3°) Decade Below fP Phase = 0° (-5.7°)

RC21fp

12

VINR

VOUT

A = VOUT/VIN

Single Zero Circuit Equivalent

1Vpx

100k

L159H

100kRs

Zeros and Bode Plots

Zero Location = fZ

Magnitude = +20dB/Decade Slope Slope begins at fZ and continues up as

frequency increases Actual Function = +3dB up @ fZ

Phase = +45°/Decade Slope through fZ

Decade Above fZ Phase = +90° (+84.3°) Decade Below fZ Phase = 0° (5.7°)

RL*2

1fz

+90

-90

+45

+-45

10 100 1k 10k 100k 1M 10M

Frequency (Hz)0

(d

egre

es)

+90o

0o

+45o/Decade

+45o @ fZ

0

20

40

60

80

100

10M1M100k10k1k100101

Frequency (Hz)

A (d

B)

fZ

+20dB/Decade+6dB/Octave

Straight-Line Approximation

G

1.414G = +3dB(1/0.707)G = +3dB Actual

Function

fp

13

Capacitor - Intuitive Model

frequencycontrolled

resistor

OPEN SHORT

DC XCHi-f XCDC < XC < Hi-f

XC = 1/(2fC)

14

Inductor - Intuitive Model

frequencycontrolled

resistor

SHORT OPEN

DC XLHi-f XLDC < XL < Hi-f

XL = 2fL

15

Capacitor and Inductor - Impedance vs FrequencyT

Capacitor ImpedanceC = 159nF Inductor Impedance

L = 159mH

Frequency (Hz)100m 1 10 100 1k 10k 100k 1M 10M

Impe

danc

e (o

hms)

100m

1

10

100

1k

10k

100k

1M

10M

Inductor ImpedanceL = 159mH

Capacitor ImpedanceC = 159nF

Capacitor and InductorImpedance vs Frequency

16

Low frequency=Low Impedance

High frequency=High Impedance

Low frequency=High Impedance

High frequency=Low Impedance

Op Amp - Intuitive Model

K(f)Ro

Rin

Vo

Vout

Vdiff+

-

IN+

IN-

x1

17

Op-Amp Loop Gain Model

VOUT/VIN = Acl = Aol/(1+Aolβ)If Aol >> 1 then Acl ≈ 1/βAol: Open Loop Gainβ: Feedback FactorAcl: Closed Loop Gain

VOUT

VFB

RF

RI

b=VFB/VOUT

bnetwork

b

Aol+

-

VOUTVIN

+

-

RF

RI

VIN

+

-

bnetwork

VFB

VOUTAol

18

19

VOUT

VFB

RF

RI

b=VFB/VOUT

bnetwork

+

-

RF

RI

VIN

+

-

bnetwork

VFB

VOUT

β is easy to calculate as feedback network around the Op Amp 1/β is reciprocal of β Easy Rules-Of-Thumb and Tricks to Plot 1/β on Op Amp Aol Curve Plotting Aol Curve and 1/β Curve shows Loop Gain

b and 1/b

Amplifier Stability CriteriaVOUT/VIN = Aol / (1+ Aolβ)If: Aolβ = -1 Then: VOUT/VIN = Aol / 0 ∞

If VOUT/VIN = ∞ Unbounded Gain

Any small changes in VIN will result in large changes in VOUT which will feed back to VIN and result in even larger changes in VOUT OSCILLATIONS INSTABILITY !!

Aolβ: Loop GainAolβ = -1 Phase shift of +180°, Magnitude of 1 (0dB)fcl: frequency where Aolβ = 1 (0dB)

Stability Criteria:At fcl, where Aolβ = 1 (0dB), Phase Shift < +180°Desired Phase Margin (distance from +180° Phase Shift) > 45°

20

21

b

Aol+

-

VOUTVIN

Op Amp Loop Gain ModelOp Amp is “Closed Loop”

Loop Gain Test:(An Open Loop Test)Break the Closed Loop at b

Ground VIN

Inject AC Source, VTest, into b

Aolβ = VOUT

b

Aol+

-

VOUTVIN

+

-VTest

Traditional Loop Gain Test

22

Op Amp Loop Gain ModelOp Amp is “Closed Loop”

VOUT/VIN = Aol / (1+Aolb)

SPICE Loop Gain Test:Op Amp Loop Gain Test is an “Open Loop” TestSPICE finds a DC Operating Point before it does an AC

Analysis so loop must be closed for DC and open for AC.

Break the Closed Loop at VOUT

Ground VIN source impedance low for AC analysis

Inject: AC Source, VTest, into RF(Inject: AC Source into High Impedance Node)Read: Aolβ = Loop Gain = VOUT

(Read: Loop Gain from Low Impedance Node)

+

-

RF

RI

VIN

+

-

bnetwork

VFB

VOUT

+

-

VTest

1TF

1TH

Short for ACOpen for DC

Open for ACShort for DC

Aol+

-

RF

RI

VIN

+

-

bnetwork

VFB

VOUTAol

Traditional Loop Gain Test

-

++

4

3

51

2

U1 OPA2376V1 2.5V

V2 2.5V

VOUTRI 1kOhm

RF 10kOhm

LT 1TH

CT 100nF

+

VG1

VFB

VF1 -279.24uV

-25.38uV

-279.24uV

-

++

4

3

51

2

U1 OPA2376V1 2.5V

V2 2.5V

VOUTRI 1kOhm

RF 10kOhm

+

VG1

VFB CT Open

LT Short

-

++

4

3

51

2

U1 OPA2376V1 2.5V

V2 2.5V

VOUTRI 1kOhm

RF 10kOhm

+

VG1

VFB CT Short

LT Open

23

SPICE Loop Gain Test

Loop Gain (Aolb) = VOUTb = VFB1/b = 1 / VFBAol = VOUT / VFB

DC Equivalent Circuit AC Equivalent Circuit

DC Analysis

DC Analysis DC Analysis

0

20

40

60

80

100

10M1M100k10k1k100101

Frequency (Hz)

Aol (

dB)

fcl b

Acl

Aol

Aol b(Loop Gain)

Closed Loop Response

Open Loop Response

24

Plot (dB) 1/β on Op Amp Aol (dB)Aolβ = Aol(dB) – 1/β(dB)Aolβ = Aol / (1/β) = Aolβ Note how Aolβ changes with frequency

Loop Gain (Aolb) from Aol and 1/b

25

“Rate-of-Closure” Stability Criteria using 1/β & Aol

0

20

40

60

80

100

10M1M100k10k1k100101

Frequency (Hz)

Aol (

dB)

Aol

b

b

b

b

fcl1

fcl4

fcl3

fcl2

**

*

**

*

At fcl: Loop Gain (Aolb) = 1 (0dB)

Rate-of-Closure @ fcl =(Aol slope – 1/β slope)

*20dB/decade Rate-of-Closure @ fcl = STABLE

**40dB/decade Rate-of-Closure @ fcl = UNSTABLE

26

Loop Gain (Aolb) Example

0

20

40

60

80

100

10M1M100k10k1k100101

Frequency (Hz)

A (d

B)

Aol

fcl

b

fp1

fp2fz1

Aol b

Rate-of-Closure @ fcl = 40dB/decade UNSTABLE!

+

-

+

-VIN

RI

RFCin

VOUT

10k

1k

0.15 F

STABLE

Example 1: Note locations of poles and zeros in Aol & 1/b

Aol 1/b Loop Gainfp1 pole ----- polefp2 pole ----- polefz1 ----- zero pole

0

20

40

60

80

100

10M1M100k10k1k100101

Frequency (Hz)

A (d

B)

fp1

fz1

fp2

fcl

27

To Plot Aolβ from Aol & 1/β Plot:Poles in Aol curve are Poles in Aolβ (Loop Gain)PlotZeros in Aol curve are Zeros in Aolβ (Loop Gain) Plot

Poles in 1/β curve are Zeros in Aolβ (Loop Gain) PlotZeros in 1/β curve are Poles in Aolβ ( Loop Gain) Plot[Remember: β is the reciprocal of 1/β]

Loop Gain (Aolβ) Plot from Aol & 1/β Plot

180

0

135

45

10 100 1k 10k 100k 1M 10M

Frequency(Hz)90

(d

egre

es)

-45

fp1

fz1

fp2

fcl

Loop Gain (Aolb) Phase at fcl:Phase Shift = -180Phase Margin = 0

STABLE

STABLE

Example 1: Note locations of poles and zeros in Loop Gain

Aol 1/b Loop Gainfp1 pole ----- polefp2 pole ----- polefz1 ----- zero pole

28

1/β Always = Closed Loop Response

0

20

40

60

80

100

10M1M100k10k1k100101

Frequency (Hz)

A (d

B)

b

VOUT/VIN

Aol

fcl

SSBW(Small Signal BandWidth)

VOUT/VIN = Aol/(1+Aolβ)At fcl: Aolβ = 1 VOUT/VIN = Aol/(1+1) ~ Aol No Loop Gain left to correct for errors VOUT/VIN follows the Aol curve at f > fcl

Note:

1/β is the AC, Small Signal, Closed Loop, ”Noise Gain” for the Op Amp.

VOUT/VIN is often NOT the same as 1/β.

+

-

RI

RF

VOUT

10k

100k

1kRn

Cn16nF

+

-VIN

VNOISE

29

How to Modify 1/β for Stable Circuits

+

-

+

-VIN

VOUT

RFRI

Rn Cn RpCp

ZIINPUT Network

ZFFEEDBACK Network

30

1/β “First Order Analysis” for ZF

+

-

+

-VIN

VOUT

RFRI

RpCp

100k1k

10k1.59nF

1/β Low Frequency = RF/RI = 100 40dBCp = Open at Low Frequency

1/β High Frequency = (Rp//RF)/RI ≈ Rp/RI = 10 20dBCp = Short at High Frequency

Pole in 1/β when Magnitude of XCp = RF

Magnitude XCp = 1/(2∙п∙f∙Cp)

fp = 1/(2∙п∙RF∙Cp) = 1kHz Zero in 1/β when Magnitude of XCp = Rp

fz = 1/(2∙п∙Rp∙Cp) = 10kHz )]RF//RI(Rp[Cp21fz

)RpRF(Cp21fp

:equationsExact ZF

T

Aol

1/b

fz

fp

Frequency (Hz)1 10 100 1k 10k 100k 1M 10M

Gai

n (d

B)

-40

-20

0

20

40

60

80

100

120

140

fp

fz

Aol

1/b

ZF Network (fp and fz)Aol and 1/b

31

TINA SPICE: 1/β for ZF

Lo fHi f

1st Order Actualfp 1kHz 917.020Hzfz 10kHz 9.038kHz

1st Order ActualLo f 40dB 40.086dBHi f 20dB 20.079dB

+

-

+

-VIN

VOUT

RFRI

RpCp

100k1k

10k1.59nF

32

+

-

+

-VIN

VOUT

RFRI

Rn Cn

100k

1k

10k

15.9nF

1/β Low Frequency = RF/RI = 10 20dBCn = Open at Low Frequency

1/β High Frequency = RF/(RI//Rn) ≈ RF/Rn =100 40dBCn = Short at High Frequency

Zero in 1/β when Magnitude of XCn = RI

Magnitude XCn = 1/(2∙п∙f∙Cn)

fz = 1/(2∙п∙RI∙Cn) = 1kHz Pole in 1/β when Magnitude of XCn = Rn

fp = 1/(2∙п∙Rn∙Cn) = 10kHz

1/β “First Order Analysis” for ZI

)]RF//RI(Rn[Cn21fz

RnCn21fp

:equationsExact ZI

T

Aol

1/b

fz

fp


Gai

n (d

B)

-40

-20

0

20

40

60

80

100

120

140

fp

fz

Aol

1/b

ZI Network (fp and fz)Aol and 1/b

33

TINA SPICE: 1/β for ZI

Lo f

Hi f

1st Order Actualfz 1kHz 999.496Hzfp 10kHz 9.935kHz

1st Order ActualLo f 20dB 20.828dBHi f 40dB 40.906dB

+

-

+

-VIN

VOUT

RFRI

Rn Cn

100k

1k

10k

15.9nF

Stability Analysis - Method 1 (Loaded Aol & 1/b Technique)

(Riso Compensation)

V+

V-

+

-

+U1 OPA627E

Vo

R2 100kR3 4.99k

+

Vin CLoad 1u

35

Capacitive Loading on Op Amp Outputs

T

Time (s)0.00 150.00u 300.00u

V1

0.00

20.00m

40.00m

VG1

0.00

1.00m

0 150u 300uTime (seconds)

40m

0

Vo (V)

Vin (V)

20m

01m

Unity Gain Buffer Circuits Circuits with Gain

V+

V-

+

Vin

+

-

+

U1 OPA627E

Vo

CLoad 1uF

0 150u 300uTime (seconds)

80m

0

Vo (V)

T

Time (s)0.00 150.00u 300.00u

VF1

-40.00m

-10.00m

20.00m

50.00m

80.00m

VG1

0.00

20.00m

Vin (V)

20m

-40m20m

10m

Will this circuit behavior get you a raise in pay?

T

fp1Aol PoleLow Frequency

fp2Loaded AolAdditional Pole

-20dB/decade

-40dB/decade


Gai

n (d

B)

-80

-60

-40

-20

0

20

40

60

80

100

120

140

Rate-of-Closure40dB/decade

fcl

1/b

Loaded Aol due to CLoad

-40dB/decade

-20dB/decade



36

Loaded Aol V+

V-

V+

V-V+ 15V

V- 15V +

-

+

U1 OPA627E

VOUT

CLoad 1uF

+

Vtest

LT 1THCT 1TF

VFB

Loaded Aol = VOUT / VFBFor AC Test VFB = VtestLoaded Aol = VOUT

353.900776nV

353.900776nV

STABLE

V+

V-

V+

V-V+ 15

V- 15 +

-

+

U1 OPA627E

VOUT

CLoad 1u

+

Vtest

LT 1TCT 1T

VFB

Loaded Aol = VOUT / VFBFor AC Test VFB = VtestLoaded Aol = VOUT

37

Loaded Aol Model

Loaded AolRo 54

CLoad 1u

+

Vtest

-

+

-

+

Aol 1M

LT Open

CT Short

+

Aol

Ro 54

CLoad 1u

Loaded Aol

38

Loaded Aol ModelT

Gai

n (d

B)

-80.00

-60.00

-40.00

-20.00

0.00


Pha

se [d

eg]

-90.00

-45.00

0.00

0

-20

-40

-60

-800

-45

-901 10 100 1k 10k 100k 1M 10M 100M

Gai

n (d

B)

Pha

se (d

egre

es)

Frequency (Hz)

Loaded AOLPole

+

Aol

Ro 54

CLoad 1u

Loaded Aol

fp2

CLoadRo212fp

Equation Pole AolLoaded

39

Loaded Aol Model

+

=

T

Gai

n (d

B)

-80.00

-60.00

-40.00

-20.00

0.00


Pha

se [d

eg]

-90.00

-45.00

0.00

0

-20

-40

-60

-800

-45

-901 10 100 1k 10k 100k 1M 10M 100M

Gain

(dB)

Phas

e (de

gree

s)

Frequency (Hz)

T

Gai

n (d

B)

-40.00

-20.00

0.00

20.00

40.00

60.00

80.00

100.00

120.00


Pha

se [d

eg]

0.00

45.00

90.00

135.00

180.00

120

806040200

-20-40180

135

90

45

01 10 100 1k 10k 100k 1M 10M 100M

Gain

(dB)

Phas

e (d

egre

es)

Frequency (Hz)

100

T

Vol

tage

(V)

-40

-20

0

20

40

60

80

100

120


Vol

tage

(V)

0.00

45.00

90.00

135.00

180.00

120

80604020

0-20-40180

135

90

45

01 10 100 1k 10k 100k 1M 10M 100M

Gain

(dB)

Phas

e (de

grees

)

Frequency (Hz)

100

Aol Aol Load

Loaded Aol

fp1

fp1

fp2

fp2

Note: Addition on Bode Plots = Linear Multiplication

T

Gai

n (d

B)

-80

-60

-40

-20

0

20

40

60

80

100

120

140


Pha

se [d

eg]

-45

0

45

90

135

180

Gain : VOUT A:(222.74k; -32.46f)

Phase : VOUT A:(222.74k; 548.41m)

fcl

Loaded AolLoop Gain & Phase

a

40

Loaded Aol – Loop Gain & Phase

V+

V-

V+

V-V+ 15V

V- 15V +

-

+

U1 OPA627E

VOUT

CLoad 1uF

+

Vtest

LT 1THCT 1TF

VFB

Loop Gain (Aolb) = VOUT

Phase Margin at fclSTABLE

41

Riso Compensation

V+

V-

V+

V-V+ 15V

V- 15V

+

-

+U1 OPA627E VOUT

CLoad 1uF+

VIN

Riso 6Ohm

VOA

Riso will add a zero in the Loaded Aol Curve

T



fz1Loaded AolRiso CompensationAdditional Zero

-20dB/decade

-20dB/decade

-40dB/decade


Gai

n (d

B)

-80

-60

-40

-20

0

20

40

60

80

100

120

140Loaded Aol with Riso Compensation

1/bRate-of-Closure20dB/decade

-40dB/decade

-20dB/decade

-20dB/decade

fz1Loaded AolRiso CompensationAdditional Zerofp2

Loaded AolAdditional Pole


fcl

42

Riso Compensation Results

V+

V-

V+

V-V+ 15V

V- 15V

+

-

+U1 OPA627E VOUT

CLoad 1uF

Riso 6Ohm

VOA

LT 1THCT 1TF

+

Vtest

Loaded Aol = VOA

STABLE

43

Riso Compensation Theory

VOUTRo 54

CLoad 1u

+

Vtest

-

+

-

+

Aol 1M

Riso 6

LT Open

CT Short

Loaded Aol

+

Aol

Ro 54

CLoad 1u

Loaded Aol

Riso 6

V+

V-

V+

V-V+ 15

V- 15 +

-

+

U1 OPA627E

VOUT

CLoad 1u+

Vtest

LT 1T

CT 1T

Riso 6

VOA

T

Gai

n (d

B)

-40.00

-20.00

0.00


Pha

se [d

eg]

-90.00

-45.00

0.00

0

-20

-400

-45

-901 10 100 1k 10k 100k 1M 10M 100M

Gai

n (d

B)

Pha

se (d

egre

es)

Frequency (Hz)

44

Riso Compensation Theory+

Aol

Ro 54

CLoad 1u

Loaded Aol

Riso 6

sCLoadRiso)(Ro1sRisoCLoad1 (s)Loaded Aol

Function Transfer

CLoadRiso)(Ro21 2fp

:Pole

CLoadRiso21 1fz

:Zero

fp2fz1

45

Riso Compensation TheoryT

Gai

n (d

B)

-40.00

-20.00

0.00

20.00

40.00

60.00

80.00

100.00

120.00


Pha

se [d

eg]

0.00

45.00

90.00

135.00

180.00

120

806040200

-20-40180

135

90

45

01 10 100 1k 10k 100k 1M 10M 100M

Gain

(dB)

Phas

e (d

egre

es)

Frequency (Hz)

100T

Gai

n (d

B)

-40.00

-20.00

0.00


Pha

se [d

eg]

-90.00

-45.00

0.00

0

-20

-400

-45

-901 10 100 1k 10k 100k 1M 10M 100M

Gain

(dB)

Phas

e (de

gree

s)

Frequency (Hz)

T

Gai

n (d

B)

-40.00

-20.00

0.00

20.00

40.00

60.00

80.00

100.00

120.00


Pha

se [d

eg]

0.00

45.00

90.00

135.00

180.00

120

80604020

0-20-40180

135

90

45

01 10 100 1k 10k 100k 1M 10M 100M

Gain

(dB)

Phas

e (de

grees

)

Frequency (Hz)

100

+

=

Aol

Aol Load

Loaded Aolfp2 fz1

fp1

fp1

fp2 fz1

Note: Addition on Bode Plots = Linear Multiplication

Riso Compensation Design Steps1) Determine fp2 in Loaded Aol due to CLoad

A) Measure in SPICE with CLoad on Op Amp Output

2) Plot fp2 on original Aol to create new Loaded Aol

3) Add Desired fz2 on to Loaded Aol Plot for Riso CompensationA) Keep fz1 < 10*fp2 (Case A)B) Or keep the Loaded Aol Magnitude at fz1 > 0dB (Case B) (fz1>10dB will allow for Aol variation of ½ Decade in Unity Gain Bandwidth)

4) Compute value for Riso based on plotted fz1

5) SPICE simulation with Riso for Loop Gain (Aolb) Magnitude and Phase

6) Adjust Riso Compensation if greater Loop Gain (Aolb) phase margin desired

7) Check closed loop AC response for VOUT/VINA) Look for peaking which indicates marginal stabilityB) Check if closed AC response is acceptable for end application

8) Check Transient response for VOUT/VIN A) Overshoot and ringing in the time domain indicates marginal stability B) Determine if settling time is acceptable for end application

46

T

-40dB/decade

Case B: CLoad=2.9nF

-20dB/decade

Case A: CLoad=1uF

fp2Case BCLoad=2.9nF

fp2Case ACLoad=1uF

Vol

tage

(V)

-80

-60

-40

-20

0

20

40

60

80

100

120

140


Vol

tage

(V)

-45

0

45

90

135

180


fp2Case ACLoad=1uF

Case A: CLoad=1uF

Case B: CLoad=2.9nF

-40dB/decade

-20dB/decade

Loaded Aol Case A: CLoad = 1uFCase B: Cload = 2.9nF

VOA[1] 2.9n[F] A:(983.366787k; 21.799287) VOA[1] 2.9n[F] A:(983.366787k; 45)

VOA[2] 1u[F] B:(2.980143k; 71.980276) VOA[2] 1u[F] B:(2.980143k; 45)

ab

1),2) Loaded Aol and fp2

47

Case A, CLoad=1uF, fp2=2.98kHzCase B, CLoad=2.9nF, fp2=983.37kHz

V+

V-

V+

V-V+ 15V

V- 15V

+

-

+U1 OPA627E VOUT

CLoad 2.9nF

Riso 0Ohm

VOA

LT 1THCT 1TF

+

Vtest

Loaded Aol = VOA

3) Add fz1 on Loaded Aol

48

T


fz1Case BCLoad=2.9nF

fp2Case ACLoad=1uF

fz1Case ACLoad=1uF


Vol

tage

(V)

-80

-60

-40

-20

0

20

40

60

80

100

120

140

Loaded AolAdd Riso Compensation

4.07MHz

fz1Case BCLoad=2.9nF


fz1Case ACLoad=1uF

fp2Case ACLoad=1uF

983.37kHz

29.8kHz

2.98kHz

Case A, CLoad=1uF, fz1=29.8kHzCase B, CLoad=2.9nF, fz1=4.07MHz

4) Compute Value for Riso

49

Case A, CLoad=1uF, fz1=29.8kHzCase B, CLoad=2.9nF, fz1=4.07MHz

CLoad1fz21Riso

CLoadRiso21 1fz

:Zero

5.36Ω use

:29.8kHzfz1 F,1CLoad A,Case Zero,

34.5F1kHz8.292

1Riso

CLoadRiso21 1fz

13.7Ω use

:4.07MHzfz1 2.9nF,CLoad B, Case Zero,

48.13nF9.2MHz07.42

1Riso

CLoadRiso21 1fz

T

VOA

-20

0

20

40

60

80

100

120

140


VOA

0

45

90

135

180fcl

Loop GainCase A: CLoad=1uF

VOA: VOA A:(1.519941M; 1.364794f)

VOA: VOA A:(1.519941M; 87.522388)

a

5),6) Loop Gain, Case A

50

Phase Margin at fcl = 87.5 degrees

V+

V-

V+

V-V+ 15V

V- 15V

+

-

+U1 OPA627E VOUT

CLoad 1uF

Riso 5.36Ohm

VOA

LT 1THCT 1TF

+

Vtest

Loop Gain (Aolb) = VOA

5),6) Loop Gain, Case B

51

T

VOA

-20

0

20

40

60

80

100

120

140


VOA

0

45

90

135

180fcl

Loop GainCase B: CLoad=2.9nF

VOA: VOA A:(4.48315M; -1.528291f)

VOA: VOA A:(4.48315M; 54.167774)

a

Phase Margin at fcl = 54 degrees

V+

V-

V+

V-V+ 15V

V- 15V

+

-

+U1 OPA627E VOUT

CLoad 2.9nF

Riso 13.7Ohm

VOA

LT 1THCT 1TF

+

Vtest

Loop Gain (Aolb) = VOA

52

7) AC VOUT/VIN, Case A

T VOA-3dB=1.58MHz

VOUT-3dB=30.44kHz

Gai

n (d

B)

-80

-60

-40

-20

0

20


Pha

se [d

eg]

-180

-135

-90

-45

0

VOUT/VINRiso CompensationCase A, CLoad=1uF

VOA-3dB=1.58MHz

VOUT-3dB=30.44kHz

V+

V-

V+

V-V+ 15V

V- 15V

+

-

+U1 OPA627E VOUT

CLoad 1uF

Riso 5.36Ohm

VOA

+

VIN

53

8) Transient Analysis, Case A

T

Time (s)0 500u 1m 2m 2m

VIN

-10.00m

10.00m

VOA

-10.27m

10.27m

VOUT

-10.01m

10.01m

VOUT / VINTransient AnalysisCase A, CLoad=1uF

V+

V-

V+

V-V+ 15V

V- 15V

+

-

+U1 OPA627E VOUT

CLoad 1uF

Riso 5.36Ohm

VOA

+

VIN

V+

V-

V+

V-

V+ 15V

V2 15V

+

-

+

U1 OPA627E

VOUT

CLoad 1uF

Riso 6Ohm

VOA

VIN 5V RLoad 200Ohm

A+

ILoad 24.271845mA

5V

4.854369V

54

Riso Compensation: Key Design Consideration

V+

V-

V+

V-

V+ 15V

V2 15V

+

-

+

U1 OPA627E

VOUT

CLoad 1uF

Riso 6Ohm

VOA

VIN 5VRLoad 1kOhm

A+

ILoad 4.970179mA

5V

4.970179V

Accuracy of VOUT depends on Load Current Light Load Current

Heavy Load Current

Stability Analysis - Method 2 (Aol and1/bTechnique)

(CF Compensation)

56

Large Input Resistance &Input Capacitance

T

Time (s)990.00u 1.01m 1.03m 1.05m

VIN

-10.00m

10.00m

VOUT

-26.95m

27.04m

V1 18V

V2 18V

VOUT

RF 180kOhmRI 180kOhm

-

+ +U1 OPA140

+

VIN

Do you want this hidden in your product - in production?

T

Aol

1/b


Vol

tage

(V)

-40

-20

0

20

40

60

80

100

120

140

fz1104kHz

Aol and 1/b

Rate-of-Closure 40dB/decade

fcl

1/b

Aol

57

Aol and 1/b

V1 18V

V2 18V

LT 1TH

CT 1TF

Vout

+ VG1


VFB

-

+ +U1 OPA140

Aol = Vout/VFB1/b = 1/VFBLoop Gain (Aolb) = Vout

STABLE

58

Op Amp Input Capacitance

-

+ +

U1 OPA140

VEE 18V

VOUT

VCC 18V

Ccm- 7pF

Ccm+ 7pF

Cdiff 10pF

IN-

IN+

OPA140 - Input Capacitance

59

Equivalent Input Capacitance andb

-

+ +U2 OPA140

V1 18V

VOUT

V2 18V

Ccm- 7pF

Ccm+ 7pF

Cdif f 10pF

RI 180kOhm RF 180kOhm+

VIN

-

+ +U3 OPA140

V3 18V

VOUT

V4 18V

Cin_eq 17pF

RI 180kOhm RF 180kOhm

VFB

RI 180kOhm

RF 180kOhm

VOUT

Cin_eq 17pF

VFB

VFBVOUT1VOUTVFB

b

b

kHz104k)180k // 180(pF17π2

1(RF // RI)Cin_eqπ2

1 1 zero: fzβ1

dB62k180k1801

RIRF1

RIRIRF DC

β1

RI

RIRFeq_Cin

RIRFRIRFeq_Cin

1 s

β1

RI // X)X (RI // RF

β1

Cin_eq

Cin_eq

tion)simplifica (after

:nComputatio 1/β

60

Equivalent Input Capacitance andb

RI 180kOhm

RF 180kOhm

VOUT

Cin_eq 17pF

VFB

(Set to 1V)

b

CF Compensation Design Steps1) Determine fz1 in 1/b due to Cin_eq

A) Measure in SPICE ORB) Compute by Datasheet CDIFFand CCM and Circuit RF and RI

2) Plot 1/b with fz1 on original Aol

3) Add Desired fp1 on 1/b for CF CompensationA) Keep fp1 < 10*fz1B) Keep fp1 < 1/10 * fcl

4) Compute value for CF based on plotted fp1

5) SPICE simulation with CF for Loop Gain (Aolb) Magnitude and Phase

6) Adjust CF Compensation if greater Loop Gain (Aolb) phase margin desired

7) Check closed loop AC response for VOUT/VINA) Look for peaking which indicates marginal stabilityB) Check if closed AC response is acceptable for end application

8) Check Transient response for VOUT/VIN A) Overshoot and ringing in the time domain indicates marginal stability

61

62

1),2),3) Plot Aol, 1/b, Add fp1 in 1/b for Stability

T

Aol

1/b


Gai

n (d

B)

-40

-20

0

20

40

60

80

100

120

140

Lo-f = 6dBHi-f = 15dB

Aol and 1/bInput Capacitance Compensation

1/b

Aol

New fcl

Add fp1316kHz

fz1104kHz

For fp1:fp1 < 10 * fz1fp1 < 1/10 * fcl

kHz48.327k180pF7.2π2

1 RFCFπ2

1

kHz77.89pF7.2//pF17)k180//k180(CF//Cin_eq)RI//RF(

dB62k180k1801

RIRF1

RIRIRF

(

CFRF1sCF

CFCin_eqRIRFRIRF

1 s CFCin_eq

β1

))//

fp1 :pole β1

21

21 fz1 :zero

β1

DC β1

tion)simplifica after

X // RIX // (RI X(RF

β1

:nComputatio 1/β

Cin_eq

Cin_eqCF

63

4) Compute value of CF

-

+ +U3 OPA140

V3 18V

VOUT

V4 18V

RI 180kOhm RF 180kOhm

Cin_eq 17pF

CF 2.7pF

+

VIN

RI 180kOhm

RF 180kOhm

VOUT

Cin_eq 17pF

VFB

CF 2.7pF

T

Vout

-20

0

20

40

60

80

100

120

140


Vout

0

45

90

135

180

Loop GainCF Compensation

Vout: Vout A:(1.308649M; 4.921411f)

Vout: Vout A:(1.308649M; 68.191331)

fcl

a

5), 6) Loop Gain Check

64

Phase Margin at fcl = 68 degrees

V1 18V

V2 18V

LT 1

TH

CT 1TF

Vout

+

Vtest


VFB

-

+ +U1 OPA140

CF 2.7pF

Aol = Vout/VFB1/b = 1/VFBLoop Gain (Aolb) = Vout

65

7) VOUT/VIN AC Response

T

VOUT-3dB=394.5kHz

Gai

n (d

B)

-60

-40

-20

0


Pha

se [d

eg]

0

45

90

135

180

VOUT/VINCF Compensation VOUT

-3dB=394.5kHz

V1 18V

V2 18V

VOUT


-

+ +U1 OPA140

+

VIN

CF 2.7pF

T

Time (s)0 500u 1m 2m 2m

VIN

-10.00m

10.00m

VOUT

-9.98m

10.11m

VOUT / VINTransient Analysis

66

8) Transient Analysis

V1 18V

V2 18V

VOUT


-

+ +U1 OPA140

+

VIN

CF 2.7pF

Documents

Solving Op Amp Stability Issues