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Lecture18

# Lecture18 - Here the open loop gain is determined from the...

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V1 15 AC I1 1 V3 0 V4 0 R1 100 R2 1K R3 1K C1 3p U1 LT1001 R5 1K R6 1K C2 3p V5 -15 U2 LT1001 R4 100 V2 1 AC X Y +V +V -V -V +V -V .ac dec 30 .1 100Meg Elements added to compute current gain Gi=I(V3)/I(V4) Element added to compute voltage gain Gv=-V(x)/V(y) Here the open loop gain is determined from the closed loop system[1]. The voltage gain and current gain are independently measured in two circuits identical except for the elements inserted into the loop to make the measurements. Then the open loop gain can be plotted by plotting the quantity: ((I(V3)/I(V4))*(-V(x)/V(y))-1)/((I(V3)/I(V4))+(-V(x)/V(y))+2) Alternatively, you can add the lines .func Gi()=I(V3)/I(V4) .func Gv()=-V(x)/V(y) .func middlebrook(x,y) = (x*y-1)/(x+y+2) to your plot.defs file and then plot the quantity middlebrook(Gi(),Gv()) This example schematic is supplied for informational/educational purposes only. 1] Middlebrook, R.D., "Measurement of Loop Gain in Feedback Systems", Int. J. Electronics, vol 38, No. 4, pp. 485-512, 1975 See the example file LoopGain2.asc for an improved technique.

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