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1
What causes amplifier stability issues???
2
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 (
dB)
-20dB/Decade-6dB/Octave
fPG
0.707G = -3dB
ActualFunction
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 = -84.3°
Decade Below fP Phase = -5.7°
3
Zeros and Bode Plots
+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 (
dB)
fZ
+20dB/Decade+6dB/Octave
Straight-Line Approximation
G
1.414G = +3dB(1/0.707)G = +3dB Actual
Function
R
C
VOUT
A = VOUT/VIN
Single Zero Circuit Equivalent
X100,000
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 = +84.3°
Decade Below fZ Phase = 5.7°
4
Capacitor Intuitive Model
frequencycontrolled
resistor
OPEN SHORT
DC XCHi-f XCDC < XC < Hi-f
XC = 1/(2fC)
5
Inductor Intuitive Model
frequencycontrolled
resistor
SHORT OPEN
DC XLHi-f XLDC < XL < Hi-f
XL = 2fL
6
Op-Amp Intuitive Model
K(f)Ro
Rin
Vo
Vout
Vdiff+
-
IN+
IN-
x1
7
Op-Amp Loop Gain Model
+
-
RF
RI
VIN
+
-
network
Aol+
-
VOUTVIN
VFB
VOUT
VFB
RF
RI
=VFB/VOUT
VOUT
network
VOUT/VIN = Acl = Aol/(1+Aolβ)
If Aol >> 1 then Acl ≈ 1/β
Aol: Open Loop Gain
β: Feedback Factor
Acl: Closed Loop Gain
8
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 Gain
Aolβ = -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°
9
T
Time (s)
1.95m 2.08m 2.20m
Vo
-15.00
0.00
15.00
Vfb
-2.25
0.00
2.25
VG1
0.00
10.00m
ab
Fundamental Cause of Amplifier Stability Issues
• Too much delay in the feedback network
-
+BUF
BUF BUF
R1 100kR2 100k+ VG1
Vfb
Vo
DELAY
DELAY
10
T
Time (s)
1.95m 2.23m 2.50m
VG1
0.00
10.00m
Vfb
-37.08m
62.12m
Vo
-1.00
1.16
Cause of Amplifier Stability Issues• Example circuit with too much delay in the feedback network
-
+BUF
BUF BUF
R1 100kR2 100k+
VG1
Vfb
Vo
R3 10
C1 10uC2 20p
11
Cause of Amplifier Stability Issues• Real circuit translation of too much delay in the feedback network
-
+BUF
BUF BUF
R1 100kR2 100k
+
VG1
Vfb
VoRo 10
Cin 4pCstray 16p
Cload 10u
12
Cause of Amplifier Stability Issues• Same results as the example circuit
-
+BUF
BUF BUF
R1 100kR2 100k
+
VG1
Vfb
VoRo 10
Cin 4pCstray 16p
Cload 10u
T
Time (s)
1.95m 2.23m 2.50m
VG1
0.00
10.00m
Vfb
-37.08m
62.12m
Vo
-1.00
1.16
13
How do we determine if our system has too
much delay??
14
Phase Margin• Phase Margin is a measure of the “delay” in the loop
V+
V-
+
VG1 353.901124n+
-
+
U2 OPA627E
VF1
Open-Loop
T
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[de
g]
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)
Pha
se (
degr
ees)
Frequency (Hz)
100
Phase Margin
AOL
AOLPhase
Unity-Gain f(cl)
15
Small-Signal Overshoot vs. Phase Margin
From: Dorf, Richard C. Modern Control Systems. Addison-Wesley Publishing Company. Reading, Massachusetts. Third Edition, 1981.
Phase Margin Overshoot
90° 0
80° 2%
70° 5%
60° 10%
50° 16%
40° 25%
30° 37%
20° 53%
10° 73%
16
Damping Ratio vs. Phase Margin
From: Dorf, Richard C. Modern Control Systems. Addison-Wesley Publishing Company. Reading, Massachusetts. Third Edition, 1981.
17
AC Peaking vs. Damping Ratio
From: Dorf, Richard C. Modern Control Systems. Addison-Wesley Publishing Company. Reading, Massachusetts. Third Edition, 1981.
Phase Margin AC Peaking @Wn
90° -7dB
80° -5dB
70° -4dB
60° -1dB
50° +1dB
40° +3dB
30° +6dB
20° +9dB
10° +14dB
18
T
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00120
80
60
40
20
0
-20
-40
Gai
n (d
B)
100
1 10 100 1k 10k 100k 1M 10M 100MFrequency (Hz)
AOL
AOL*B
1/Beta
Rate of Closure= 20dB/decade
Rate of ClosureRate of Closure: Rate at which 1/Beta and AOL intersect
ROC = Slope(1/Beta) – Slope(AOL)
ROC = 0dB/decade – (-20dB/decade) = 20dB/decade
19
T
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00120
80
60
40
20
0
-20
-40
Gai
n (d
B)
100
1 10 100 1k 10k 100k 1M 10M 100MFrequency (Hz)
AOL
1/Beta
Rate of Closure and Phase MarginSo a pole in AOL or a zero in 1/Beta inside the loop will decrease AOL*B Phase!!
40dB/decade
40dB/decade
AOL pole
1/B zero
20
Rate of Closure and Phase MarginRelationship between the AOL and 1/Beta rate of closure and Loop-Gain (AOL*B) phase margin
T
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[d
eg
]
0.00
45.00
90.00
135.00
180.00
a120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)
Pha
se (d
egre
es)
Frequency (Hz)
100
Phase Margin≥ 45 degrees!
Rate of Closure= 20dB/decade
AOL
1/B
AOL*BPhase
AOL*B
21
Rate of Closure and Phase Margin
AOL Pole
1/Beta Zero
22
Testing for Rate of Closure in SPICE
V+
V-
+
VG1 0
+
-
+
U2 OPA627E
VF1
R1 1k R2 1k
V+
V-
+
-
+
U1 OPA627E
Vo
R3 1k R4 1k
L1 1T
C1 1T
+
VG2
Vfb
VinShort out the input source
Break the loop with L1 at the inverting input
Inject an AC stimulus through C1
• Break the feedback loop and inject a small AC signal
23
Breaking the Loop
DC AC
V-
V+
+
-
+U1 OPA627E
Vo
Rf 1k
Rg 1k
+
VG2 Vfb
Vin
L1
C1V-
V+
+
-
+U1 OPA627E
Vo
Rf 1kRg 1k+
VG1
Vfb
VinL1
C1
24
Plotting AOL, 1/Beta, and Loop GainAOL = Vo/Vin
1/Beta = Vo/Vfb
AOL*B = Vfb/Vin
T
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 14.14k 200.00M
Ph
ase
[d
eg
]
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)
Pha
se (d
egre
es)
Frequency (Hz)
100
Phase Margin= 82degrees
AOL*B
1/B
AOL*BPhase
AOLV+
V-
+
-
+
U1 OPA627E
Vo
+
VG1 0
L1 1T
C1 1T
Vin
Rg 1k Rf 1k
Vfb
25
Noise Gain• Understanding Noise Gain vs. Signal Gain
Inverting Gain, G = -1 Non-Inverting Gain, G = 2
Both circuits have a NOISE GAIN (NG) of 2.
NG = 1 + ΙGΙ = 2 NG = G = 2
V+
V-
V+
V-
+
VG1
+
-
+
U1 OPA627E
Vo
R1 1k R2 1k
+
VG1
+
-
+
U1 OPA627E
Vo
R1 1k R2 1k
26
Noise Gain• Noise Gain vs. Signal Gain
Gain of -0.1V/V, Is it Stable?
Inverting Gain, G = -0.1
If it’s unity-gain stable then it’s stable as an inverting attenuator!!!
V+
V-
+
VG1
+
-
+
U1 OPA627E
Vo
R1 10k R2 1k
V+
V-
+
VG1
+
-
+
U1 OPA627E
Vo
R1 10k R2 1k
Noise Gain, NG = 1.1
27
Capacitive Loads
28
Capacitive LoadsUnity Gain Buffer Circuits Circuits with Gain
V+
V-
+
-
+
U1 OPA627E
VF1
R2 100kR3 249
+
VG1 0 CLoad 1uV+
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
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
29
Capacitive Loads – Unity Gain Buffers - ResultsDetermine the issue:
Pole in AOL!!
ROC = 40dB/decade!!
Phase Margin 0!!
NG = 1V/V = 0dB
V+
V-
+
-
+
U1 OPA627E
Vo
CLoad 1u+
VG1 0
L1 1T
C1
1T
Vin
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)
Pha
se (
degr
ees)
Frequency (Hz)
100
T
Vo
ltag
e (
V)
-40
-20
0
20
40
60
80
100
120
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Vo
ltag
e (
V)
0.00
45.00
90.00
135.00
180.00
Phase Margin= 0.2degrees!
Rate of Closure= 40dB/decade!
AOL + AOL*B
1/B
AOL*BPhase
Pole in AOL
30
Capacitive Loads – Unity Gain Buffers - Theory
V+
V-
+
-
+
U1 OPA627E
Vo
CLoad 1u+
VG1
L1 1T
C4
1T
Vin
Loaded AOLRo 54
CLoad 1u
L1
+
VG1
-
+
-
+
AOL 1M
C1
+
AOL
Ro 54
CLoad 1u
Loaded AOL
31
Capacitive Loads – Unity Gain Buffers - Theory
Transfer function:
W(s)=1
1+RoCload
s
T
Ga
in (
dB
)
-80.00
-60.00
-40.00
-20.00
0.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[de
g]
-90.00
-45.00
0.00
0
-20
-40
-60
-80
0
-45
-901 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)
Pha
se (
degr
ees)
Frequency (Hz)
Loaded AOLPole
CLoadRoFPOLE
2
1
+
Vin
Ro 54
CLoad 1u
Loaded AOL
32
Capacitive Loads – Unity Gain Buffers - Theory
X
=
T
Ga
in (
dB
)
-80.00
-60.00
-40.00
-20.00
0.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[d
eg
]
-90.00
-45.00
0.00
0
-20
-40
-60
-80
0
-45
-901 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)Ph
ase
(deg
rees
)
Frequency (Hz)
T
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[d
eg
]
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)Ph
ase
(deg
rees
)
Frequency (Hz)
100
T
Vo
lta
ge
(V
)
-40
-20
0
20
40
60
80
100
120
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Vo
lta
ge
(V
)
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gain
(dB)
Phas
e (de
gree
s)
Frequency (Hz)
100
AOL AOL Load
Loaded AOL
33
Stabilize Capacitive Loads – Unity Gain Buffers
34
Unity-Gain circuits can only be stabilized by modifying the AOL load
T
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00120
80
60
40
20
0
-20
-40
Gai
n (d
B)
100
1 10 100 1k 10k 100k 1M 10M 100MFrequency (Hz)
Stability Options
35
Method 1: Riso
V+
V-
+
-
+
U1 OPA627E CLoad 1u
Riso 6+
VG1
Vo
VLoad
36
Method 1: Riso - ResultsTheory: Adds a zero to the Loaded AOL response to cancel the pole
T
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[de
g]
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gai
n (
dB)
Pha
se (
degr
ees)
Frequency (Hz)
100
Phase Margin = 87.5degrees!
Rate of Closure = 20dB/decade
AOL + AOL*B
Pole in AOL1/B
AOL*BPhase
Zero in AOL
V+
V-
+
-
+
U1 OPA627E CLoad 1u
Riso 6
+
VG1
Vo
VLoad
37
Method 1: Riso - ResultsWhen to use: Works well when DC accuracy is not important, or when loads are very light
Vo (V)
Vload (V)
Vin (V)
T
Time (s)
0.00 125.00u 250.00u
V1
0.00
20.37m
V2
0.00
20.00m
VG1
0.00
20.00m
V+
V-
+
-
+
U1 OPA627E CLoad 1u
Riso 6
+
VG1
Vo
VLoad
38
Method 1: Riso - Theory
V+
V-
+
-
+
U1 OPA627E CLoad 1u
L1 1T
C1 1T
Vin
Riso 5
+
VG1
Vo
Ro 54
CLoad 1u
+
VG1
-
+
-
+
AOL 1M
Riso 5
Loaded AOL
C1
Ro 54
Riso 5
Loaded AOL
CLoad 1u
+
AOL
39
Method 1: Riso - Theory
Zero Equation:
f(zero)=1
2piRisoCLoad
s
Pole Equation:
f(pole)=1
2pi(Ro+Riso)CLoad
s
Transfer function:
Loaded AOL(s)=1+CLoad
Risos
1+(Ro+Riso)CLoad
s
Ro 54Ohm
Riso 5Ohm
Loaded AOL
CLoad 1uF
+
Vin
T
Ga
in (
dB
)-40.00
-20.00
0.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[de
g]
-90.00
-45.00
0.00
0
-20
-40
0
-45
-901 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)
Pha
se (
degr
ees)
Frequency (Hz)
40
Method 1: Riso - Theory
X
=
T
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[d
eg
]
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)Ph
ase
(deg
rees
)
Frequency (Hz)
100T
Ga
in (
dB
)
-40.00
-20.00
0.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[d
eg
]
-90.00
-45.00
0.00
0
-20
-40
0
-45
-901 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)Ph
ase
(deg
rees
)
Frequency (Hz)
T
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[d
eg
]
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gain
(dB)
Phas
e (d
egre
es)
Frequency (Hz)
100
41
Method 1: Riso - DesignEnsure Good Phase Margin:
1.) Find: fcl and f(AOL = 20dB)2.) Set Riso to create AOL zero: Good: f(zero) = Fcl for PM ≈ 45 degrees. Better: f(zero) = F(AOL = 20dB) will yield slightly less than 90 degrees phase margin
fcl = 222.74kHz
f(AOL = 20dB) = 70.41kHz
T
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
f(AOL = 20dB)
fcl
120
80
60
40
20
0
-20
-40
Gai
n (
dB)100
1 10 100 1k 10k 100k 1M 10M 100MFrequency (Hz)
42
Method 1: Riso - Design
T
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[de
g]
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)
Pha
se (
degr
ees)
Frequency (Hz)
100
F(zero) = 70.41kHz
PM = 84°
T
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[de
g]
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gai
n (
dB)
Pha
se (
degr
ees)
Frequency (Hz)
100
F(zero) = 222.74kHz
PM = 52°
f(AOL = 20dB) = 70.41kHz
→ Riso = 2.26Ohms
fcl = 222.74kHz
→ Riso = 0.715Ohms
Zero Equation:
f(zero)=1
2piRisoCLoad
s
Pole Equation:
f(pole)=1
2pi(Ro+Riso)CLoad
s
Transfer function:
Loaded AOL(s)=1+CLoad
Risos
1+(Ro+Riso)CLoad
s
2.26R
V+
V-
+
-
+
U1 OPA627E
CLoad 1u
L1 1T
C1 1T
Vin
Riso 714m
+
VG1
Vo
1uF
0.715R
V+
V-
+
-
+
U1 OPA627E
CLoad 1u
L1 1T
C1 1T
Vin
Riso 714m
+
VG1
Vo
1uF
Ensure Good Phase Margin: Test
43
Method 1: Riso - Design
T
Ga
in (
dB)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[de
g]
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Ga
in (
dB
)P
ha
se (
de
gre
es)
Frequency (Hz)
100
PM_min = 35°
F(zero) = 26.5kHzF(pole) =
2.65kHz
T
Gai
n (d
B)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Pha
se [d
eg]
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Ga
in (
dB
)P
ha
se (
de
gre
es)
Frequency (Hz)
100
PM_min = 20°
F(zero) = 100.2kHz
F(pole) = 2.86kHz
Riso = Ro/9 Riso = Ro/34
Prevent Phase Dip:
Place the zero less than 1 decade from the pole, no more than 1.5 decades away Marginal: 1.5 Decades: F(zero) ≤ 35*F(pole) → Riso ≥ Ro/34 → 70° Phase Shift Desirable: 1 Decade: F(zero) ≤ 10*F(pole) → Riso ≥ Ro/9 → 55° Phase Shift
44
Method 1: Riso – Design Summary
6R
V+
V-
+
-
+
U1 OPA627E
CLoad 1u
L1 1T
C1 1T
Vin
Riso 714m
+
VG1
Vo
1uF
T
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[de
g]
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)
Pha
se (
degr
ees)
Frequency (Hz)
100
PM_min = 35°
PM = 87.5°
Final Circuit
Summary:
Ensure stability by placing Fzero ≤ 10* Fpole
45
Method 1: Riso - Disadvantage
6R
1uFV+
V-
+
-
+U1 OPA627E
CLoad 1u
Riso 6
+
VG1
Vo
Vload
RLoad 25
25R
+ -
T
Time (s)
0.00 125.00u 250.00u
Vol
tage
(V)
0.00
20.09m20.19m
0
0 125u 250u
Vo
ltag
e (
V)
Time (seconds)
Riso Voltage Drop
Vo
VLoad
Disadvantage:
Voltage drop across Riso may not be acceptable
46
Method 2: Riso + Dual Feedback
V+
V-
+
-
+
U1 OPA627E CLoad 1u
Riso 6
+
VG1
VLoad
Rf 49k
Cf 100n
Vo
47
Method 2: Riso + Dual FeedbackTheory: Features a low-frequency feedback, Rf, to cancel the Riso drop and a high-frequency feedback, Cf, to create the AOL pole and zero.
T
Ga
in (
dB
)
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Frequency (Hz)
1.00 10.00 100.00 1.00k 10.00k 100.00k 1.00M 10.00M 100.00M
Ph
ase
[de
g]
0.00
45.00
90.00
135.00
180.00
120
80
60
40
200
-20
-40
180
135
90
45
01 10 100 1k 10k 100k 1M 10M 100M
Gai
n (d
B)
Pha
se (
degr
ees)
Frequency (Hz)
100
Phase Margin = 87.5degrees!
Rate of Closure = 20dB/decade
AOL + AOL*B
Pole in AOL1/B
AOL*BPhase
Zero in AOL
V+
V-
+
-
+
U1 OPA627E CLoad 1u
+
VG1
Rf 49k
Cf 100n
VoL1 1T
C1 1T
Vfb
Vin
Riso 6
48
Method 2: Riso + Dual FeedbackWhen to Use: Only practical solution for very large capacitive loads ≥ 10uF
When DC accuracy must be preserved across different current loads
T
Time (s)
0.00 150.00u 300.00u
V1
0.00
20.27m
V2
0.00
20.00m
VG1
0.00
20.00m
0 150u 300uTime (seconds)
20.3m
0
VLoad(V)
020m
20m0
Vo(V)
Vin(V)
V+
V-
+
-
+
U1 OPA627E CLoad 1u
Riso 5
+
VG1
Vload
R2 49k
C1 100n
Vo
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