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Riso with Dual Feedback
Op Amp Stability
Collin Wells
Tim Green
Precision Linear Analog Applications
November 21, 2013
1
Riso with Dual FB Topology
(OPA330 Example)
RF 10k
FB#1
CF
VFB 2.500004V
-
Riso
VOUT 2.500005V
U1 OPA330
+
+
VOA 2.500005V
R2 10k
V2 5
R1 10k
FB#2
Vcc 5
CL
2
Zo Test
L1 1GH
Zo(dB) = VOUT
Zo(ohms) = VOUT on Logarithmic scale
V2 2.5V
C1 1GF
-
+
VOUT
+
U1 OPA330
V1 2.5V
IG1 0
AC Current Generator
AC = 1
3
Zo Test
T
4.37k
Gain (dB)
OPA330 Zo
130.34
3.89
1.00m
316.23
Frequency (Hz)
100.00M
4
Modified Aol
J1
+
C3 1T
VG1
RF 10k
Cdiff 2p
L1 1T
J1
-
+
Riso
VOA
Vdd 2.5
C2 2.2n
Vout
+
U1 OPA330
R2 76.8k
V2 2.5
R1 76.8k
Ccm 4p
CL
VOA = ModAol (Modified Aol)
5
Zo and CL
T
1.59T
ZM1[1]: 100p[F]
OPA330 Zo and CL
ZM1[2]: 1n[F]
ZM1[3]: 10n[F]
ZM1[4]: 100n[F]
Gain (dB)
ZM1[5]: 1u[F]
50.33k
Zo
1.59m
1m
10m
100m
1
10
100
1k
10k
Frequency (Hz)
100k
1M
10M
100M
Modified Aol will have Complex Poles with sharp phase shifts anywhere CL
Crosses Zo in Zo “Inductive” region.
6
Modified Aol with CL
J1
+
C3 1T
VG1
RF 10k
Cdiff 2p
L1 1T
J1
-
+
Vout 2.500009V
+
U1 OPA330
R2 76.8k
V2 5
R1 76.8k
Ccm 4p
VOA 2.500009V
Vdd 5
C2 2.2n
CL 150n
VOA = ModAol (Modified Aol)
7
Modified Aol with CL
T 140
120
100
80
Modified Aol with CL
Gain (dB)
60
40
20
Vout[5]: 100p[F]
Vout[25]: 1u[F]
0
Vout[9]: 1n[F]
-20
-40
Vout[20]: 100n[F]
Vout[15]: 10n[F]
-60
-80
-100
180
Phase [deg]
135
Vout[5]: 100p[F]
90
Vout[9]: 1n[F]
45
Vout[25]:1u[F]
0
-45
Vout[20]: 100n[F]
-90
Vout[15]: 10n[F]
-135
1.00m
316.23
Frequency (Hz)
100.00M
8
Zo, CL, Riso
T
1.59T
ZM1[1]: 100p[F]
CL
ZM1[2]: 1n[F]
OPA330 Zo, CL, Riso
CL
ZM1[3]: 10n[F]
CL
ZM1[4]: 100n[F]
CL
ZM1[5]: 1u[F]
CL
Riso=400ohms, CL=1uF
Series Combination Highest Z wins Intersects Zo as Resistive Impedance
Gain (dB)
Riso
ZM1[6]: 1k[Ohm]
50.33k
ZM1[5]: 800[Ohm]
Riso
Zo
Riso
ZM1[4]: 600[Ohm]
Riso
ZM1[2]: 200[Ohm]
Riso
ZM1[1]: 100[Ohm]
ZM1[3]: 400[Ohm]
Riso
1.59m
1m
10m
100m
1
10
100
1k
10k
Frequency (Hz)
100k
1M
10M
100M
9
Modified Aol with CL=1uF, Riso=400ohms
J1
+
C3 1T
VG1
RF 10k
Cdiff 2p
L1 1T
J1
-
+
Riso 400
Vout 2.499999V
+
U1 OPA330
VOA 2.499999V
CL 1u
R2 10k
V2 5
R1 10k
Ccm 4p
Vdd 5
C2 2.2n
VOA = ModAol (Modified Aol)
10
Modified Aol with CL=1uF, Riso=400ohms
Note:
1) Modified Aol looks like an Aol with a second pole around 1kHz and a zero at about 20kHz
2) No abrupt phase shifts and minimum phase is 29.488 degrees away from 180 degree total phase shift
3) Modified Aol UGBW=56.5kHz with phase margin at 72.33 degrees
11
FB#1 with CL=1uF, Riso=400ohms
J1
+
C3 1T
VG1
RF 10k
Cdiff 2p
L1 1T
J1
-
+
Riso 400
Vout 2.499999V
+
U1 OPA330
VOA 2.499999V
CL 1u
R2 10k
V2 5
R1 10k
Ccm 4p
Vdd 5
C2 2.2n
VOA = ModAol (Modified Aol)
12
FB#1 with CL=1uF, Riso=400ohms
T
140
FB#1 CL=1uF, Riso=400ohms
Modified Aol
120
100
80
Gain (dB)
60
FB#2
40
fz2
FB#1
20
0
Net 1/Beta
Lowest FB Wins!
-20
fcl
New fcl
-40
-60
-80
1m
10m
100m
1
10
100
1k
Frequency (Hz)
10k
100k
1M
10M
Note:
1) At fcl, where loop gain goes to zero, there is a 40dB/decade rate-of-closure indicating INSTABILITY for FB#1 alone
2) We will add FB#2 s shown for a Net 1/Beta which will be stable at new fcl
100M
13
FB#2 with CL=1uF, Riso=400ohms, RF=100k, CF=82nF
C3 1T
J1
+
FB#2=1/VFB
VFB
VG1
fz2=1/(2 pi CF RF)
RF 100k
L1 1T
CF 82n
L2 1T
-
Riso 400
Vout
+
VOA
Vdd 5
C1 1T
+
U1 OPA330
R2 10k
V2 5
R1 10k
J1
CL 1u
14
FB#2 with CL=1uF, Riso=400ohms, RF=100k, CF=82nF
T
100
80
FB2
Gain (dB)
60
40
fz2
19.46Hz
20
0
-20
1m
10m
100m
1
10
100
1k
Frequency (Hz)
10k
100k
1M
10M
100M
15
Loop Gain for CL=1uF, Riso=400ohms Final Compensation
C3 1T
J1
+
VFB=Loop Gain
VFB 2.500004V
VG1
RF 100k
C1 82n
Cdiff 2p
L1 1T
J1
-
+
Riso 400
Vout 2.500015V
+
U1 OPA330
VOA 2.500015V
CL 1u
R2 10k
V2 5
R1 10k
Ccm 4p
Vdd 5
C2 2.2n
16
Loop Gain for CL=1uF, Riso=400ohms Final Compensation
Phase
Margin
17
Riso with Dual Feedback curves
18