Difference Amplifier with Low Pass Tim Green Collin

Difference Amplifier with Low Pass Tim Green Collin Wells January 30, 2013 1

Original Circuit Analysis 2

OPA 277 Data Sheet Aol 3

OPA 277 TINA SPICE Model Aol 4

OPA 277 TINA SPICE Model Aol Macromodel looks to be incorrect for no load phase margin. Suspect No Load phase margin to be closer to 2 k load at 54 degrees. 5

OPA 277 TINA SPICE Model Zo 6

OPA 277 TINA SPICE Model Zo 7

Original Difference Amp 8

Original Difference Amp – Vin 1 Xfer 9

Original Difference Amp – Vin 1 Xfer Peaking in the closed loop gain is indicative of marginal stability and undesired gain of noise in the peaking region. 10

Original Difference Amp – Vin 2 Xfer 11

Original Difference Amp – Vin 2 Xfer Peaking in the closed loop gain is indicative of marginal stability and undesired gain of noise in the peaking region. 12

Original Difference Amp – Vin 1 & Vin 2 Xfer Peaking in the closed loop gain is indicative of marginal stability and undesired gain of noise in the peaking region. 13

Original Difference Amp – Vin 2 Xfer w/o C 3 14

Original Difference Amp – Vin 2 Xfer w/o C 3 Without C 3 the feedback capacitor C 1 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. . 15

Original Difference Amp – Loop Gain 16

Original Difference Amp – Loop Gain At 0 d. B Loop Gain Phase amrgin is only 4. 7 degrees. 17

Original Difference Amp – Modified Aol & 1/Beta Cap load on output of op amp is equivalent to C 1 and C 3 in series or 50 n. F and Zo of 40 ohms gives second pole fp 2 in modified Aol. 1/Beta goes to 6 d. B at high frequency and at fcl rate-of-closure is 40 d. B/decade Indicating stability problem. 18

New Circuit Analysis 19

New Difference Amp – Vin 1 & Vin 2 Xfer New circuit will fix stability problem and give good xfer function for Vin 1 and Vin 2. Split resistor on VIn 2 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 C 3. 20

New Difference Amp – Vin 1 & Vin 2 Xfer New Xfer curves for Vin 1 and Vin 2 show excellent symmetry out to very high frequency with no sharp peaking. 21

New Difference Amp – Loop Gain 22

New Difference Amp – Loop Gain Macromodel looks to be incorrect for no load phase margin. Suspect No Load phase margin to be closer to 2 k load at 54 degrees And real New Circuit to have 54 degees of phase margin. 23

New Difference Amp – Modified Aol & 1/Beta Modified Aol is still about 20 d. B/decade crossing 0 d. B. 1/Beta intersection is about 20 d. B/decade indicating good stability. 24

Further Reading on Zo and Stability http: //www. en-genius. net/site/zones/acquisition. ZONE/technical_notes/acqt_050712 25