Difference amplifier with low pass
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Difference Amplifier with Low Pass. Tim Green Collin Wells January 30, 2013. Original Circuit Analysis. OPA277 Data Sheet Aol. OPA277 TINA SPICE Model Aol. OPA277 TINA SPICE Model Aol. Macromodel looks to be incorrect for no load phase margin.

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Difference Amplifier with Low Pass

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Difference amplifier with low pass

Difference Amplifier with Low Pass

Tim Green

Collin Wells

January 30, 2013


Original circuit analysis

Original Circuit Analysis


Opa277 data sheet aol

OPA277 Data Sheet Aol


Opa277 tina spice model aol

OPA277 TINA SPICE Model Aol


Opa277 tina spice model aol1

OPA277 TINA SPICE Model Aol

Macromodel looks to be incorrect for no load phase margin.

Suspect No Load phase margin to be closer to 2k load at 54 degrees.


Opa277 tina spice model zo

OPA277 TINA SPICE Model Zo


Opa277 tina spice model zo1

OPA277 TINA SPICE Model Zo


Original difference amp

Original Difference Amp


Original difference amp vin1 xfer

Original Difference Amp – Vin1 Xfer


Original difference amp vin1 xfer1

Original Difference Amp – Vin1 Xfer

Peaking in the closed loop gain is indicative of marginal stability

and undesired gain of noise in the peaking region.


Original difference amp vin2 xfer

Original Difference Amp – Vin2 Xfer


Original difference amp vin2 xfer1

Original Difference Amp – Vin2 Xfer

Peaking in the closed loop gain is indicative of marginal stability

and undesired gain of noise in the peaking region.


Original difference amp vin1 vin2 xfer

Original Difference Amp – Vin1 & Vin2 Xfer

Peaking in the closed loop gain is indicative of marginal stability

and undesired gain of noise in the peaking region.


Original difference amp vin2 xfer w o c3

Original Difference Amp – Vin2 Xfer w/o C3


Original difference amp vin2 xfer w o c31

Original Difference Amp – Vin2 Xfer w/o 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..


Original difference amp loop gain

Original Difference Amp – Loop Gain


Original difference amp loop gain1

Original Difference Amp – Loop Gain

At 0dB Loop Gain Phase amrgin is only 4.7 degrees.


Original difference amp modified aol 1 beta

Original Difference Amp – Modified Aol & 1/Beta

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.


New circuit analysis

New Circuit Analysis


New difference amp vin1 vin2 xfer

New Difference Amp – Vin1 & Vin2 Xfer

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.


New difference amp vin1 vin2 xfer1

New Difference Amp – Vin1 & Vin2 Xfer

New Xfer curves for Vin1 and Vin 2 show excellent symmetry out to very

high frequency with no sharp peaking.


New difference amp loop gain

New Difference Amp – Loop Gain


New difference amp loop gain1

New Difference Amp – Loop Gain

Macromodel looks to be incorrect for no load phase margin.

Suspect No Load phase margin to be closer to 2k load at 54 degrees

And real New Circuit to have 54 degees of phase margin.


New difference amp modified aol 1 beta

New Difference Amp – Modified Aol & 1/Beta

Modified Aol is still about 20dB/decade crossing 0dB.

1/Beta intersection is about 20dB/decade indicating good stability.


Further reading on zo and stability

Further Reading on Zo and Stability

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


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