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..

16

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

17

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.

18

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.

19

New Circuit Analysis

20

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.

22

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.

25

Further Reading on Zo and Stability

http://www.en-genius.net/site/zones/acquisitionZONE/technical_notes/acqt_050712