Riso with Dual Feedback Op Amp Stability Collin

Riso with Dual Feedback Op Amp Stability Collin Wells Tim Green Precision Linear Analog Applications November 21, 2013 1

Riso with Dual FB Topology (OPA 330 Example) 2

Zo Test 3

Zo Test 4

Modified Aol 5

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

Modified Aol with CL 7

Modified Aol with CL 8

Zo, CL, Riso CL CL CL Riso=400 ohms, CL=1 u. F Series Combination Highest Z wins Intersects Zo as Resistive Impedance Riso Riso 9

Modified Aol with CL=1 u. F, Riso=400 ohms 10

Modified Aol with CL=1 u. F, Riso=400 ohms Note: 1) Modified Aol looks like an Aol with a second pole around 1 k. Hz and a zero at about 20 k. Hz 2) No abrupt phase shifts and minimum phase is 29. 488 degrees away from 180 degree total phase shift 3) Modified Aol UGBW=56. 5 k. Hz with phase margin at 72. 33 degrees 11

FB#1 with CL=1 u. F, Riso=400 ohms 12

FB#1 with CL=1 u. F, Riso=400 ohms FB#2 fz 2 Net 1/Beta Lowest FB Wins! New fcl Note: 1) At fcl, where loop gain goes to zero, there is a 40 d. B/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 13

FB#2 with CL=1 u. F, Riso=400 ohms, RF=100 k, CF=82 n. F 14

FB#2 with CL=1 u. F, Riso=400 ohms, RF=100 k, CF=82 n. F 15

Loop Gain for CL=1 u. F, Riso=400 ohms Final Compensation 16

Loop Gain for CL=1 u. F, Riso=400 ohms Final Compensation Phase Margin 17

Riso with Dual Feedback curves 18