Version 4 SHEET 1 2412 1728 WIRE 224 64 224 48 WIRE 528 160 528 144 WIRE -80 176 -128 176 WIRE 128 176 -80 176 WIRE 224 176 224 144 WIRE 224 176 208 176 WIRE 240 176 224 176 WIRE 272 176 240 176 WIRE 496 176 352 176 WIRE 656 192 560 192 WIRE 768 192 736 192 WIRE 880 192 768 192 WIRE -80 208 -80 176 WIRE 496 208 464 208 WIRE 768 224 768 192 WIRE 528 240 528 224 WIRE -368 288 -368 272 WIRE 768 304 768 288 WIRE -80 320 -80 288 WIRE -368 384 -368 368 WIRE -128 384 -128 176 WIRE 512 384 -128 384 WIRE 880 384 880 192 WIRE 880 384 592 384 WIRE -368 480 -368 464 WIRE -256 480 -256 464 WIRE 528 528 528 512 WIRE -80 544 -128 544 WIRE 128 544 -80 544 WIRE 144 544 128 544 WIRE 256 544 224 544 WIRE 496 544 256 544 WIRE -80 560 -80 544 WIRE 656 560 560 560 WIRE 768 560 736 560 WIRE 880 560 768 560 WIRE -368 576 -368 560 WIRE -256 576 -256 560 WIRE 496 576 464 576 WIRE 768 592 768 560 WIRE 528 608 528 592 WIRE -80 656 -80 640 WIRE 768 672 768 656 WIRE -128 752 -128 544 WIRE 512 752 -128 752 WIRE 880 752 880 560 WIRE 880 752 592 752 FLAG 224 48 0 FLAG -256 576 0 FLAG -80 320 0 FLAG -80 656 0 FLAG 128 544 X FLAG 256 544 Y FLAG 528 144 +V FLAG -256 464 +V FLAG -368 576 0 FLAG -368 464 -V FLAG 528 240 -V FLAG 528 512 +V FLAG 528 608 -V FLAG 240 176 Z FLAG 464 208 offs FLAG 464 576 offs FLAG -368 384 0 FLAG -368 272 offs FLAG 768 304 0 FLAG 768 672 0 SYMBOL voltage -256 464 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 2.5 SYMBOL current 224 64 R0 WINDOW 38 39 48 Left 2 SYMATTR SpiceModel AC SYMATTR InstName I1 SYMATTR Value 1 SYMBOL voltage 224 176 R90 WINDOW 0 -32 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName V3 SYMATTR Value 0 SYMBOL voltage 256 176 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 -32 56 VBottom 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V4 SYMATTR Value 0 SYMBOL res -96 192 R0 SYMATTR InstName R2 SYMATTR Value 5k SYMBOL res 608 368 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 5k SYMBOL res -96 544 R0 SYMATTR InstName R5 SYMATTR Value 5k SYMBOL res 608 768 M270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R6 SYMATTR Value 5k SYMBOL voltage -368 464 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V5 SYMATTR Value -2.5 SYMBOL voltage 240 544 R90 WINDOW 0 -32 56 VBottom 2 WINDOW 3 32 56 VTop 2 WINDOW 38 60 56 VTop 2 SYMATTR InstName V2 SYMATTR Value 1 SYMATTR SpiceModel AC SYMBOL voltage -368 272 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V6 SYMATTR Value 0 SYMBOL Opamps\\LTC6248 528 192 R0 SYMATTR InstName U1 SYMBOL Opamps\\LTC6248 528 560 R0 SYMATTR InstName U2 SYMBOL res 640 208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R1 SYMATTR Value 100 SYMBOL cap 752 224 R0 SYMATTR InstName C1 SYMATTR Value 10n SYMBOL res 640 576 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R4 SYMATTR Value 100 SYMBOL cap 752 592 R0 SYMATTR InstName C2 SYMATTR Value 10n TEXT -472 688 Left 2 !.ac dec 30 .01 100Meg TEXT 120 -80 Left 2 ;Elements added to \ncompute current gain\nGi=I(V3)/I(V4) TEXT 80 400 Left 2 ;Element added to \ncompute voltage gain\nGv=-V(x)/V(y) TEXT -464 -472 Left 2 ;Here the open loop gain is determined from the closed loop system[1].\nThe voltage gain and current gain are independently measured in\ntwo circuits identical except for the elements inserted into\nthe loop to make the measurements. Then the open loop gain\ncan be plotted by plotting the quantity:\n \n((I(V3)/I(V4))*(-V(x)/V(y))-1)/((I(V3)/I(V4))+(-V(x)/V(y))+2)\n \nAlternatively, you can add the lines\n.func Gi()=I(V3)/I(V4)\n.func Gv()=-V(x)/V(y)\n.func middlebrook(x,y) = (x*y-1)/(x+y+2)\nto your plot.defs file and then plot the quantity\nmiddlebrook(Gi(),Gv()) TEXT -312 848 Left 2 ;This example schematic is supplied for informational/educational purposes only. TEXT -504 888 Left 2 ;1] Middlebrook, R.D., "Measurement of Loop Gain in Feedback Systems", Int. J. Electronics, vol 38, No. 4, pp. 485-512, 1975 TEXT -464 -664 Left 2 ;This is exactly a copy from Frank Wiedmann's loopgain work.\nThanks Frank for your example.\n \nIf a single supply is used, especially take care of the input and \noutput voltage. The opamp has no gain if the output is at 0V.\nPlot this: ((I(V3)/I(V4))*(-V(x)/V(y))-1)/((I(V3)/I(V4))+(-V(x)/V(y))+2) LINE Normal 364 242 364 17 LINE Normal 114 242 364 242 LINE Normal 114 17 364 17 LINE Normal 114 242 114 17 LINE Normal 61 475 311 475 LINE Normal 61 633 311 633 LINE Normal 311 633 311 475 LINE Normal 61 633 61 475