Version 4 SHEET 1 2412 1728 WIRE 384 80 384 64 WIRE 96 160 96 144 WIRE -80 176 -128 176 WIRE 64 176 -80 176 WIRE 288 192 128 192 WIRE 384 192 384 160 WIRE 384 192 368 192 WIRE 400 192 384 192 WIRE 432 192 400 192 WIRE 560 192 512 192 WIRE 672 192 640 192 WIRE 784 192 672 192 WIRE -80 208 -80 176 WIRE 64 208 32 208 WIRE 672 224 672 192 WIRE 96 240 96 224 WIRE -368 288 -368 272 WIRE 672 304 672 288 WIRE -80 320 -80 288 WIRE -368 384 -368 368 WIRE -128 384 -128 176 WIRE 80 384 -128 384 WIRE 784 384 784 192 WIRE 784 384 160 384 WIRE -368 480 -368 464 WIRE -256 480 -256 464 WIRE 96 528 96 512 WIRE -80 544 -128 544 WIRE 64 544 -80 544 WIRE -80 560 -80 544 WIRE 336 560 128 560 WIRE 352 560 336 560 WIRE 464 560 432 560 WIRE 560 560 464 560 WIRE 672 560 640 560 WIRE 784 560 672 560 WIRE -368 576 -368 560 WIRE -256 576 -256 560 WIRE 64 576 32 576 WIRE 672 592 672 560 WIRE 96 608 96 592 WIRE -80 656 -80 640 WIRE 672 672 672 656 WIRE -128 752 -128 544 WIRE 80 752 -128 752 WIRE 784 752 784 560 WIRE 784 752 160 752 FLAG 384 64 0 FLAG -256 576 0 FLAG -80 320 0 FLAG -80 656 0 FLAG 336 560 X FLAG 464 560 Y FLAG 96 144 +V FLAG -256 464 +V FLAG -368 576 0 FLAG -368 464 -V FLAG 96 240 -V FLAG 96 512 +V FLAG 96 608 -V FLAG 400 192 Z FLAG 32 208 offs FLAG 32 576 offs FLAG -368 384 0 FLAG -368 272 offs FLAG 672 304 0 FLAG 672 672 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 384 80 R0 WINDOW 38 39 48 Left 2 SYMATTR SpiceModel AC SYMATTR InstName I1 SYMATTR Value 1 SYMBOL voltage 384 192 R90 WINDOW 0 -32 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName V3 SYMATTR Value 0 SYMBOL voltage 416 192 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 176 368 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 176 768 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 448 560 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 96 192 R0 SYMATTR InstName U1 SYMBOL Opamps\\LTC6248 96 560 R0 SYMATTR InstName U2 SYMBOL res 544 208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R1 SYMATTR Value 100 SYMBOL cap 656 224 R0 SYMATTR InstName C1 SYMATTR Value 10n SYMBOL res 544 576 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R4 SYMATTR Value 100 SYMBOL cap 656 592 R0 SYMATTR InstName C2 SYMATTR Value 10n TEXT -472 688 Left 2 !.ac dec 30 .01 100Meg TEXT 272 -40 Left 2 ;Elements added to \ncompute current gain\nGi=I(V3)/I(V4) TEXT 272 416 Left 2 ;Element added to \ncompute voltage gain\nGv=-V(x)/V(y) TEXT -456 -272 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 -456 -496 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 524 258 524 33 LINE Normal 274 258 524 258 LINE Normal 274 33 524 33 LINE Normal 274 258 274 33 LINE Normal 269 491 519 491 LINE Normal 269 649 519 649 LINE Normal 519 649 519 491 LINE Normal 269 649 269 491