MATLAB: Help with MATLAB code; discrepancy between graph’s max and Matlab’s max

matlab discrepancy

Hi,
For some reason, when I run this MATLAB code, I'm getting what looks like about 10,000 for Rf for the largest Vo on the graph, but matlab is giving me Rf at (Vo)max as 100,001
Any ideas?
clc; clear; close all;
syms Rf
Rf = 0:0.1:100000;
VRt25 = 18.*(10000)./(10000 + Rf); VRf25 = 18.*(Rf)./(10000 + Rf);
VRt2505 = 18.*(9977.81)./(9977.81 + Rf); VRf2505 = 18.*(Rf)./(9977.81 + Rf);
V_difference25 = abs(VRt25 – VRf25);
V_difference2505 = abs(VRt2505 – VRf2505);
Vo_difference = abs(V_difference25 – V_difference2505);
figure;
plot(Rf,Vo_difference)
title('Voltage Difference Vo vs. Rf')
ylabel('Vo Difference [V]')
xlabel('Rf [ohms]')
hold on
%Maximum Vo Difference
[Vo_difference_max,Rf] = max(Vo_difference)
Thank you all!

Best Answer

The max() function returns the index of Vo_difference, not Rf directly. Plus Rf starts at 0 not 1. Try this:
Rf = 0 : 0.1 : 100000;
VRt25 = 18.*(10000)./(10000 + Rf); VRf25 = 18.*(Rf)./(10000 + Rf);
VRt2505 = 18.*(9977.81)./(9977.81 + Rf); VRf2505 = 18.*(Rf)./(9977.81 + Rf);
V_difference25 = abs(VRt25 - VRf25);
V_difference2505 = abs(VRt2505 - VRf2505);
Vo_difference = abs(V_difference25 - V_difference2505);
figure;
plot(Rf,Vo_difference)
title('Voltage Difference Vo vs. Rf')
ylabel('Vo Difference [V]')
xlabel('Rf [ohms]')
hold on
%Maximum Vo Difference
[Vo_difference_max, indexAtMax] = max(Vo_difference)
RfAtMax = Rf(indexAtMax)
message = sprintf('The Rf at max Vo = %f.', RfAtMax);
helpdlg(message);
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