I have greatly simplified the SPICE model of the tester, to see what can be done to get it stable. Included circuit is open loop and adjusted for (theoretical) DC stability at the preferred operating point. Note the AC gain/phase seems to look OK, but when closed (Vqq drives R1 instead of V2), it oscillates like crazy. How to fix?
Version 4 SHEET 1 1548 688 WIRE 832 -464 592 -464 WIRE 1088 -464 832 -464 WIRE 1088 -384 1088 -464 WIRE 1472 -384 1088 -384 WIRE 1088 -352 1088 -384 WIRE 832 -272 832 -464 WIRE 832 -128 832 -192 WIRE 1088 0 1088 -272 WIRE 1120 0 1088 0 WIRE 1248 0 1200 0 WIRE 832 16 832 -48 WIRE 848 64 816 64 WIRE 944 64 928 64 WIRE 592 80 592 -464 WIRE 1072 112 736 112 WIRE 1152 112 1072 112 WIRE 1376 112 1152 112 WIRE 544 160 32 160 WIRE 1056 160 896 160 WIRE 1152 160 1152 112 WIRE 1088 176 1088 0 WIRE 1120 176 1088 176 WIRE 896 192 896 160 WIRE 1248 192 1248 0 WIRE 1248 192 1184 192 WIRE 1280 192 1248 192 WIRE 944 208 944 64 WIRE 944 208 928 208 WIRE 1120 208 1088 208 WIRE 592 224 592 176 WIRE 848 224 784 224 WIRE 864 224 848 224 WIRE 944 240 928 240 WIRE 960 240 944 240 WIRE 1248 240 1248 192 WIRE 1248 240 1040 240 WIRE 736 256 736 112 WIRE 736 256 112 256 WIRE 1376 272 1376 112 WIRE 784 304 784 224 WIRE 784 304 640 304 WIRE 896 304 896 256 WIRE 1072 304 1072 112 WIRE 1072 304 896 304 WIRE 320 320 272 320 WIRE 480 320 320 320 WIRE 480 336 480 320 WIRE 816 336 816 64 WIRE 816 336 736 336 WIRE 1056 336 1056 160 WIRE 1152 336 1152 224 WIRE 1152 336 1056 336 WIRE 1200 336 1152 336 WIRE 32 352 32 160 WIRE 192 352 32 352 WIRE 320 352 320 320 WIRE 848 352 848 224 WIRE 864 352 848 352 WIRE 944 352 944 240 WIRE 112 368 112 256 WIRE 32 384 32 352 WIRE 80 384 32 384 WIRE 736 384 736 336 WIRE 816 384 816 336 WIRE 1152 384 1152 336 WIRE 1376 384 1376 352 WIRE 192 400 192 352 WIRE 192 400 144 400 WIRE 80 416 32 416 WIRE 480 432 480 416 WIRE 32 480 32 416 WIRE 272 480 272 320 WIRE 272 480 32 480 WIRE 736 528 736 464 WIRE 816 528 816 464 WIRE 816 528 736 528 WIRE 1088 528 1088 208 WIRE 1088 528 816 528 WIRE 1152 528 1152 464 WIRE 1152 528 1088 528 WIRE 1152 544 1152 528 WIRE 112 576 112 432 WIRE 1200 576 1200 336 WIRE 1200 576 112 576 FLAG 832 16 0 FLAG 592 224 0 FLAG 1152 544 0 FLAG 1376 384 0 FLAG 320 432 0 FLAG 480 432 0 FLAG 1472 -384 Vreg FLAG 1280 192 Vmon FLAG 640 304 Vqq SYMBOL voltage 1376 256 R0 WINDOW 0 13 4 Left 0 WINDOW 3 9 -24 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15V SYMBOL nmos 544 80 R0 WINDOW 3 64 -1 Left 0 SYMATTR Value STW11NM80 SYMATTR InstName M1 SYMBOL voltage 832 -144 R0 WINDOW 0 13 4 Left 0 WINDOW 3 9 -24 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value SINE(2500 200 60 0) SYMBOL res 816 -288 R0 SYMATTR InstName R4 SYMATTR Value 1.3Meg SYMATTR SpiceLine pwr=10 SYMBOL res 1072 -368 R0 SYMATTR InstName R14 SYMATTR Value 20Meg SYMBOL voltage 1152 480 R180 WINDOW 0 13 4 Left 0 WINDOW 3 9 -24 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V4 SYMATTR Value 15V SYMBOL res 1216 -16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R15 SYMATTR Value 16.7K SYMBOL voltage 816 480 R180 WINDOW 0 -68 56 Left 0 WINDOW 3 -88 12 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V5 SYMATTR Value 1.000 SYMBOL res 944 256 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R16 SYMATTR Value 10K SYMBOL Opamps\\LT1028A 1152 128 R0 WINDOW 3 14 138 Left 0 SYMATTR InstName U2 SYMBOL Opamps\\LT1028A 896 288 R180 WINDOW 3 -37 149 Left 0 SYMATTR InstName U3 SYMBOL Opamps\\LT1028A 112 336 R0 SYMATTR InstName U4 SYMBOL res 832 80 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R18 SYMATTR Value 10K SYMBOL res 720 368 R0 WINDOW 3 -55 103 Left 0 WINDOW 0 -34 19 Left 0 SYMATTR Value 100K SYMATTR InstName R19 SYMBOL res 336 448 R180 WINDOW 0 -45 73 Left 0 WINDOW 3 -65 35 Left 0 SYMATTR InstName R1 SYMATTR Value 20K SYMBOL res 848 368 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R2 SYMATTR Value 23.6Meg SYMBOL voltage 480 320 R0 WINDOW 0 -68 56 Left 0 WINDOW 3 40 60 Left 0 WINDOW 123 40 88 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 4.508 SYMATTR Value2 AC 1mV 0 TEXT 440 480 Left 0 !.ac dec 10 1 1e6 TEXT 1128 -240 Left 0 ;Emulates "typical" DUT TEXT 1264 48 Left 0 ;(-1.000v) TEXT 1096 -360 Left 0 ;R=(E/I)=(1200V/60uA)=20Meg] TEXT 1096 -56 Left 0 ;R=E/I=(1V/60uA)=16.67K TEXT 1136 -432 Left 0 ;n001