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Riso with Dual Feedback Op Amp Stability Collin Wells Tim Green Precision Linear Analog Applications November 21,

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Riso with Dual FB Topology (OPA330 Example) 2

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Zo Test 3

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Modified Aol 5

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Zo and CL 6 Modified Aol will have Complex Poles with sharp phase shifts anywhere CL Crosses Zo in Zo “Inductive” region.

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Modified Aol with CL 7

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Zo, CL, Riso 9 CL Riso Riso=400ohms, CL=1uF Series Combination - Highest Z wins - Intersects Zo as Resistive Impedance

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Modified Aol with CL=1uF, Riso=400ohms 10

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Modified Aol with CL=1uF, Riso=400ohms 11 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 degrees away from 180 degree total phase shift 3)Modified Aol UGBW=56.5kHz with phase margin at degrees

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FB#1 with CL=1uF, Riso=400ohms 12

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FB#1 with CL=1uF, Riso=400ohms 13 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 FB#2 Net 1/Beta Lowest FB Wins! New fcl fz2

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FB#2 with CL=1uF, Riso=400ohms, RF=100k, CF=82nF 14

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FB#2 with CL=1uF, Riso=400ohms, RF=100k, CF=82nF 15

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Loop Gain for CL=1uF, Riso=400ohms Final Compensation 16

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Loop Gain for CL=1uF, Riso=400ohms Final Compensation 17 Phase Margin

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Riso with Dual Feedback curves 18

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