Riso with Dual FeedbackOp Amp Stability
Collin Wells
Tim Green
Precision Linear Analog Applications
November 21, 20131
Riso with Dual FB Topology (OPA330 Example)
2
Vcc 5 CL
R1
10
kR
2 1
0k
Riso
RF 10k
CF
VOUT
VOA
+
-
+
U1 OPA330V2 5
VFB
FB#1
FB#2
2.500004V
2.500005V
2.500005V
Riso Dual FB Topology.TSC
Zo Test
3
V1 2.5V
V2 2.5V
VOUT
L1 1GH
C1 1GF
IG1 0
+
-
+U1 OPA330
AC Current Generator
AC = 1
Zo(dB) = VOUT
Zo(ohms) = VOUT on Logarithmic scale
OPA330 Zo Test.TSC
Zo Test
4
T
Frequency (Hz)
1.00m 316.23 100.00M
Ga
in (
dB
)
3.89
130.34
4.37k
OPA330 Zo
Modified Aol
5
J1
J1
+
-
+
U1 OPA330
Vdd 2.5 CL
V2 2.5
R1
76.8
kR
2 76
.8k
Cdi
ff 2p
Ccm 4pL1 1T
Riso
RF 10k
C2 2.2n
Vout
VOA
+
VG1
C3 1T
VOA = ModAol (Modif ied Aol)
ModAol.TSC
T
Zo
ZM1[5]: 1u[F]
ZM1[4]: 100n[F]
ZM1[3]: 10n[F]
ZM1[2]: 1n[F]
ZM1[1]: 100p[F]
Frequency (Hz)
1m 10m 100m 1 10 100 1k 10k 100k 1M 10M 100M
Ga
in (
dB
)
1.59m
50.33k
1.59T
ZM1[5]: 1u[F]
ZM1[4]: 100n[F]
ZM1[3]: 10n[F]
ZM1[2]: 1n[F]
ZM1[1]: 100p[F]OPA330 Zo and CL
Zo
Zo and CL
6
Modified Aol will have Complex Poles with sharp phase shifts anywhere CLCrosses Zo in Zo “Inductive” region.
Modified Aol with CL
7
J1
J1
+
-
+
U1 OPA330
Vdd 5 CL 150n
V2 5
R1
76.8
kR
2 76
.8k
Cdi
ff 2p
Ccm 4pL1 1T
RF 10k
C2 2.2n
Vout
VOA
+
VG1
C3 1T
VOA = ModAol (Modif ied Aol)
2.500009V
2.500009V
ModAol CL.TSC
Modified Aol with CL
8
T
Vout[15]: 10n[F]
Vout[20]: 100n[F]
Vout[25]: 1u[F] Vout[5]: 100p[F]
Vout[9]: 1n[F]
Vout[15]: 10n[F]
Vout[20]: 100n[F]
Vout[25]:1u[F]
Vout[5]: 100p[F]
Vout[9]: 1n[F]
Ga
in (
dB
)
-100
-80
-60
-40
-20
0
20
40
60
80
100
120
140
Frequency (Hz)
1.00m 316.23 100.00M
Ph
ase
[de
g]
-135
-90
-45
0
45
90
135
180
Modified Aol with CL
Vout[9]: 1n[F]
Vout[5]: 100p[F]
Vout[15]: 10n[F]
Vout[20]: 100n[F]
Vout[25]:1u[F]
Vout[20]: 100n[F]
Vout[25]: 1u[F]
Vout[15]: 10n[F]
Vout[9]: 1n[F]
Vout[5]: 100p[F]
Zo, CL, Riso
9
T
Zo
ZM1[5]: 1u[F]
ZM1[4]: 100n[F]
ZM1[3]: 10n[F]
ZM1[2]: 1n[F]
ZM1[1]: 100p[F]
ZM1[1]: 100[Ohm]ZM1[2]: 200[Ohm] ZM1[3]: 400[Ohm]
ZM1[4]: 600[Ohm]
ZM1[5]: 800[Ohm]
ZM1[6]: 1k[Ohm]
Frequency (Hz)
1m 10m 100m 1 10 100 1k 10k 100k 1M 10M 100M
Ga
in (
dB
)
1.59m
50.33k
1.59T
ZM1[1]: 100[Ohm]ZM1[2]: 200[Ohm] ZM1[3]: 400[Ohm]
ZM1[4]: 600[Ohm]
ZM1[5]: 800[Ohm]
ZM1[6]: 1k[Ohm]
ZM1[5]: 1u[F]
ZM1[4]: 100n[F]
ZM1[3]: 10n[F]
ZM1[2]: 1n[F]
ZM1[1]: 100p[F]OPA330 Zo, CL, Riso
Zo
CL
CL
CL
CL
CL
Riso
Riso
Riso
RisoRiso
Riso
Riso=400ohms, CL=1uFSeries Combination -Highest Z wins -Intersects Zo as Resistive Impedance
Riso and Zo.TSC CL and Zo.TSC
Modified Aol with CL=1uF, Riso=400ohms
10
J1
J1
+
-
+
U1 OPA330
Vdd 5
V2 5
R1
10k
R2
10k
Cdi
ff 2p
Ccm 4pL1 1T
RF 10k
C2 2.2n
Vout
VOA
+
VG1
C3 1T
Riso 400
CL 1u
VOA = ModAol (Modif ied Aol)
2.499999V
2.499999V
ModAol Riso CL.TSC
Modified Aol with CL=1uF, Riso=400ohms
11
T
VOA
-80
-60
-40
-20
0
20
40
60
80
100
120
140
Frequency (Hz)
1m 10m 100m 1 10 100 1k 10k 100k 1M 10M 100M
VOA
-90
-45
0
45
90
135
180
Modified AolCL=1uF, Riso=400ohms
VOA:
VOA A:(56.508587k; 8.881784E-16) B:(3.244457k; 36.430298)
VOA:
VOA A:(56.508587k; 72.338331) B:(3.244457k; 29.488718)
a
b
Note:1)Modified Aol looks like an Aol with a second pole around 1kHz and a zero at about 20kHz2)No abrupt phase shifts and minimum phase is 29.488 degrees away from 180 degree total phase shift3)Modified Aol UGBW=56.5kHz with phase margin at 72.33 degrees
FB#1 with CL=1uF, Riso=400ohms
12
J1
J1
+
-
+
U1 OPA330
Vdd 5
V2 5
R1
10k
R2
10k
Cdi
ff 2p
Ccm 4pL1 1T
RF 10k
C2 2.2n
Vout
VOA
+
VG1
C3 1T
Riso 400
CL 1u
VOA = ModAol (Modif ied Aol)
2.499999V
2.499999V
FB_1.TSC
FB#1 with CL=1uF, Riso=400ohms
13
T
FB#1
Modified Aol
Frequency (Hz)
1m 10m 100m 1 10 100 1k 10k 100k 1M 10M 100M
Ga
in (
dB
)
-80
-60
-40
-20
0
20
40
60
80
100
120
140
fcl
FB#1
FB#1 CL=1uF, Riso=400ohmsModified Aol
Note:1)At fcl, where loop gain goes to zero, there is a 40dB/decade rate-of-closure indicating INSTABILITY for FB#1 alone2)We will add FB#2 s shown for a Net 1/Beta which will be stable at new fcl
FB#2
Net 1/BetaLowest FB Wins!
New fcl
fz2
FB#2 with CL=1uF, Riso=400ohms, RF=100k, CF=82nF
14
J1
J1
+
-
+
U1 OPA330
Vdd 5
V2 5
R1
10k
R2
10k
RF 100k
Vout
VOA
+
VG1
C3 1T
Riso 400
VFB
CF 82n
CL 1u
L1 1
T
L2 1T
C1
1T
FB#2=1/VFB
fz2=1/(2piCFRF)
FB_2 Rev A.TSC
FB#2 with CL=1uF, Riso=400ohms, RF=100k, CF=82nF
15
T
FB2
fz219.46Hz
Frequency (Hz)
1m 10m 100m 1 10 100 1k 10k 100k 1M 10M 100M
Ga
in (
dB
)
-20
0
20
40
60
80
100
fz219.46Hz
FB2
Loop Gain for CL=1uF, Riso=400ohms Final Compensation
16
J1
J1
+
-
+
U1 OPA330
Vdd 5
V2 5
R1
10k
R2
10k
Cdi
ff 2p
Ccm 4pL1 1T
RF 100k
C2 2.2n
Vout
VOA
+
VG1
C3 1T
Riso 400
CL 1u
C1 82n
VFB
VFB=Loop Gain
2.500004V
2.500015V
2.500015V
Loop Final.TSC
Loop Gain for CL=1uF, Riso=400ohms Final Compensation
17
T
29deg
VFB
-80
-60
-40
-20
0
20
40
60
80
100
120
140
Frequency (Hz)
1m 10m 100m 1 10 100 1k 10k 100k 1M 10M 100M
VFB
-90
-45
0
45
90
135
180
29deg
Loop Gain
CL=1uF, Riso=400ohms
Final Compensation
fcl
VFB:
VFB A:(56.310625k; 9.325873f)
VFB:
VFB A:(56.310625k; 72.239224)
a
Phase Margin
Riso with Dual Feedback curves
18
Riso Dual FB CW_TG RevA.tdr