1 difference amplifier with low pass tim green collin wells january 30, 2013
TRANSCRIPT
1
Difference Amplifier with Low Pass
Tim Green
Collin Wells
January 30, 2013
2
Original Circuit Analysis
3
OPA277 Data Sheet Aol
4
OPA277 TINA SPICE Model Aol
VDD VSS
VDD
VSS
OPA277
Vs+
Vs-
+
- TR
M8
TR
M1
U2 OPA277
RT 13.333k
VDD 15
VSS 15
Voa
+
VG1
L1 1T
C1 1T
RLoad 2k
5
OPA277 TINA SPICE Model Aol
T
2kohm Load
No Load
Ga
in (
dB
)
-37
-17
2
22
41
61
81
100
120
139
Frequency (Hz)
100m 1 10 100 1k 10k 100k 1M 10M
Ph
ase
[de
g]
-224
-184
-144
-103
-63
-23
No Load
2kohm Load
OPA277 Aol Gain : Voa A:(1.000913M; -4.978656f)
Phase : Voa A:(1.000913M; -126.559031)
a
Macromodel looks to be incorrect for no load phase margin.Suspect No Load phase margin to be closer to 2k load at 54 degrees.
6
OPA277 TINA SPICE Model Zo
VDD VSS
VDD
VSS
OPA277
Vs+
Vs-
+
- TR
M8
TR
M1
U2 OPA277
RT 13.333k
VDD 15
VSS 15
Voa
L1 1T
C1 1T
IG1
7
OPA277 TINA SPICE Model Zo
T
Zo
Ma
gn
itud
e (
oh
ms)
400m
4
40
Frequency (Hz)
100m 1 10 100 1k 10k 100k 1M 10M
Zo
Ph
ase
[de
g]
-45.12
-22.56
0.00
OPA277 Zo
8
Original Difference Amp
VDD VSS
VDD
VSS
OPA277
Vs+
Vs-
+
- TR
M8
TR
M1
U2 OPA277
RT 13.333k
VDD 15
VSS 15
R1 10k
R2
250k
R4 10k
R3 250k
C1 100n
C2
100n
Vin1
Vin2
Voa
VP_in
C3
100n
9
Original Difference Amp – Vin1 Xfer
VDD VSS
VDD
VSS
OPA277
Vs+
Vs-
+
- TR
M8
TR
M1
U2 OPA277
RT 13.333k
VDD 15
VSS 15
R1 10k
R2
250k
R4 10k
R3 250k
C1 100n
C2
100n
Vin1
Vin2
Voa
VP_in
C3
100n
+
VG1
10
Original Difference Amp – Vin1 Xfer
T
VP_in
-100
-80
-60
-40
-20
0
Voa
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
20
40
Frequency (Hz)
100m 1 10 100 1k 10k 100k 1M 10M
Voa
-360
-315
-270
-225
-180
-135
-90
-45
0
Vin1 Closed Loop Transfer FunctionOriginal Design
Peaking in the closed loop gain is indicative of marginal stability and undesired gain of noise in the peaking region.
11
Original Difference Amp – Vin2 Xfer
VDD VSS
VDD
VSS
OPA277
Vs+
Vs-
+
- TR
M8
TR
M1
U2 OPA277
RT 13.333k
VDD 15
VSS 15
R1 10k
R2
250k
R4 10k
R3 250k
C1 100n
C2
100n
Vin1
Vin2
Voa
VP_in
C3
100n
+
VG1
12
Original Difference Amp – Vin2 Xfer
T
Voa
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
20
40
Frequency (Hz)
100m 1 10 100 1k 10k 100k 1M 10M
Voa
-90
-45
0
45
90
135
180
Vin2 Closed Loop Transfer FunctionOriginal Design
Peaking in the closed loop gain is indicative of marginal stability and undesired gain of noise in the peaking region.
13
Original Difference Amp – Vin1 & Vin2 Xfer
T
Vin2 Original Xfer
Vin1 Original Xfer
Frequency (Hz)
100m 1 10 100 1k 10k 100k 1M 10M
Ga
in (
dB
)
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
20
40
Vin2 Original Xfer
Vin1 Original Xfer
Peaking in the closed loop gain is indicative of marginal stability and undesired gain of noise in the peaking region.
14
Original Difference Amp – Vin2 Xfer w/o C3
VDD VSS
VDD
VSS
OPA277
Vs+
Vs-
+
- TR
M8
TR
M1
U2 OPA277
RT 13.333k
VDD 15
VSS 15
R1 10k
R2
250k
R4 10k
R3 250k
C1 100n
C2
100n
Vin1
Vin2
Voa
VP_in
+
VG1
15
Original Difference Amp – Vin2 Xfer w/o C3
T
Vin2 Xfer without C3
Vin2 Xfer with C3
Frequency (Hz)
100m 1 10 100 1k 10k 100k 1M 10M
Ga
in (
dB
)
-96
-85
-73
-62
-51
-40
-28
-17
-6
5
17
28
Vin2 Xfer with C3
Vin2 Xfer without C3
Without C3 the feedback capacitor C1 connection at the op amp output sees low impedance while there is loop gain to reduce Zo, open loop output impedance. As loop gain diminishes and goes to zero the open loop output impedance of 40 ohms flattens the xfer function..
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Original Difference Amp – Loop Gain
VDD VSS
VDD
VSS
OPA277
Vs+
Vs-
+
- TR
M8
TR
M1
U2 OPA277
RT 13.333k
VDD 15
VSS 15
R1 10k
R2
250k
R4 10k
R3 250k
C1 100n
C2
100n
Vin1
Vin2
Voa
VP_in
C3
100n
L1 1T
C4 1T+VG1
Vloop
Loop Gain = Vloop
Modified Aol = Voa
1/Beta = Voa/Vloop
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Original Difference Amp – Loop Gain
T
Vloop
-80
-60
-40
-20
0
20
40
60
80
100
120
Frequency (Hz)
100m 1 10 100 1k 10k 100k 1M 10M
Vloop
-90
-45
0
45
90
135
180
Vloop: Vloop A:(303.004812k; -269.527485m)
Vloop: Vloop A:(303.004812k; 4.723419)
Loop GainOriginal Circuit
a
At 0dB Loop Gain Phase amrgin is only 4.7 degrees.
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Original Difference Amp – Modified Aol & 1/Beta
T
Modified Aol
1/Beta
Frequency (Hz)
100m 1 10 100 1k 10k 100k 1M 10M
Vo
ltag
e (
V)
-80
-60
-40
-20
0
20
40
60
80
100
120
140
fcl
fp2
Modified Aol and 1/BetaOriginal Circuit
Voa A:(79.918497k; 26.345079) oneB A:(79.918497k; 6.020605)
Modified Aol
1/Beta
a
Cap load on output of op amp is equivalent to C1 and C3 in series or 50nF. 50nF and Zo of 40 ohms gives second pole fp2 in modified Aol.1/Beta goes to 6dB at high frequency and at fcl rate-of-closure is 40dB/decade Indicating stability problem.
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New Circuit Analysis
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New Difference Amp – Vin1 & Vin2 Xfer
VDD VSS
VDD
VSS
OPA277
Vs+
Vs-
+
- TR
M8
TR
M1
U2 OPA277
RT 13.333k
VDD 15
VSS 15
R1 10k
R2
250k
R4 5k
R3 250k
C1 100n
C2
100n
Vin1
Vin2
Voa
VP_in
+
VG1
R5 5k
C3
470p
VDD VSS
VDD
VSS
OPA277
Vs+
Vs-
+
- TR
M8
TR
M1
U2 OPA277
RT 13.333k
VDD 15
VSS 15
R1 10k
R2
250k
R4 5k
R3 250k
C1 100n
C2
100n
Vin1
Vin2
Voa
VP_in
+
VG1
R5 5k
C3
470p
New circuit will fix stability problem and give good xfer function for Vin1 and Vin2.Split resistor on VIn2 side and isollate cap load on output of op amp. Only need extra roll-off at highr frequnecies where loop gain goes to zero so can reduce value of C3.
21
New Difference Amp – Vin1 & Vin2 Xfer
T
Vin2 Xfer New
Vin1 Xfer New
Frequency (Hz)
100m 1 10 100 1k 10k 100k 1M 10M
Ga
in (
dB
)
-140
-120
-100
-80
-60
-40
-20
0
20
40
Vin2 Xfer New
Vin1 Xfer New
New Xfer curves for Vin1 and Vin 2 show excellent symmetry out to veryhigh frequency with no sharp peaking.
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New Difference Amp – Loop Gain
VDD VSS
VDD
VSS
OPA277
Vs+
Vs-
+
- TR
M8
TR
M1
U2 OPA277
RT 13.333k
VDD 15
VSS 15
R1 10k
R2
250k
R3 250k
C1 100n
C2
100n
Vin1
Vin2
Voa
VP_inC
3 47
0p
L1 1T
C4 1T+VG1
Vloop
R4 5k R5 5k
Loop Gain = Vloop
Modified Aol = Voa
1/Beta = Voa/Vloop
23
New Difference Amp – Loop Gain
T
Vloop
-40
-20
0
20
40
60
80
100
120
Frequency (Hz)
100m 1 10 100 1k 10k 100k 1M 10M
Vloop
-45
0
45
90
135
180
Loop GainNew Circuit
Vloop: Vloop A:(1.666781M; 14.314938f)
Vloop: Vloop A:(1.666781M; 36.665867)
a
Macromodel looks to be incorrect for no load phase margin.Suspect No Load phase margin to be closer to 2k load at 54 degreesAnd real New Circuit to have 54 degees of phase margin.
24
New Difference Amp – Modified Aol & 1/Beta
T
New 1/Beta
New Modified Aol
Frequency (Hz)
100m 1 10 100 1k 10k 100k 1M 10M
Vo
ltag
e (
V)
-40
-20
0
20
40
60
80
100
120
140
fcl
New 1/Beta
New Modified Aol
Modified Aol is still about 20dB/decade crossing 0dB. 1/Beta intersection is about 20dB/decade indicating good stability.
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Further Reading on Zo and Stability
http://www.en-genius.net/site/zones/acquisitionZONE/technical_notes/acqt_050712