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1
OPA277 Amplifier Stability Issue
capacitive loadsIven Xu
12/14/2013
2
Capacitive LoadsUnity Gain Buffer Circuits
T
Time (s)
0.00 1.50m 3.00m
Vin
-1.00
1.00
Vout
-1.80
1.76
Vout
V- 12
V+ 12
+
Vin
-
+ +U1 OPA277
Clo
ad 2
00n
OUT
3
Capacitive Loads – Unity Gain Buffers - Results
NG = 1V/V = 0dB
04/18/23 4
METHOD#1 FOR CLOAD=200NF
T
Time (s)
0.00 500.00u 1.00m 1.50m 2.00m
VOUT
-1.00
-500.00m
0.00
500.00m
1.00
Vin
-1.00
1.00
04/18/23 5
METHOD#2 FOR CLOAD=200NF
T
Time (s)
0.00 1.00m 2.00m
VOUT
-1.01
1.01
Vin
-1.00
1.00
6
Back up
7
Method 1: Riso
V+
V-
+
-
+
U1 OPA627E CLoad 1u
Riso 6+
VG1
Vo
VLoad
8
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 (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
9
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
10
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
11
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)
12
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
13
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)
14
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
15
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 Good: 1.5 Decades: F(zero) ≤ 35*F(pole) → Riso ≥ Ro/34 → 70° Phase Shift Better: 1 Decade: F(zero) ≤ 10*F(pole) → Riso ≥ Ro/9 → 55° Phase Shift
16
Method 1: Riso - DesignPrevent Phase Dip: Ratio of Riso to Ro
If Riso ≥ 2*Ro → F(zero) = 1.5*F(pole) → ~10° Phase Shift **Almost completely cancels the pole.
Riso = Ro*2 Phase Shift vs. Riso/Ro
17
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:
1.) Ensure stability by placing Fzero ≤ F(AOL=20dB)2.) If Fzero is > 1.5 decades from F(pole) then increase Riso up to at least Ro/343.) If loads are very light consider increasing Riso > Ro for stability across all loads
18
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
19
Method 2: Riso + Dual Feedback
V+
V-
+
-
+
U1 OPA627E CLoad 1u
Riso 6
+
VG1
VLoad
Rf 49k
Cf 100n
Vo
20
Method 2: Riso + Dual FeedbackTheory: Features a low-frequency feedback to cancel the Riso drop and a high-frequency feedback 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
21
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
22
Method 2: Riso + Dual Feedback - 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 margin3.) Set Rf so Rf >>Riso Rf ≥ (Riso * 100)4.) Set Cf ≥ (200*Riso*Cload)/Rf
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 (d
B)
100
1 10 100 1k 10k 100k 1M 10M 100MFrequency (Hz)
23
Method 2: Riso + Dual Feedback - SummaryEnsure Good Phase Margin (Same as “Riso” Method):
1.) Set Riso so f(zero) = F(AOL = 20dB)2.) Set Rf: Rf ≥ (Riso * 100) 3.) Set Cf: Cf ≥ (200*Riso*Cload)/Rf
V+
V-
+
-
+
U1 OPA627E CLoad 1u
Riso 5
+
VG1
Vload
R2 49k
C1 100n
Vo
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!
24
Thank you!!
