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

Question

David 2014-2-16

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回答： David 2014-2-16

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!

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Answer 1

Image Analyst 2014-2-16

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https://ww2.mathworks.cn/matlabcentral/answers/116303-help-with-matlab-code-discrepancy-between-graph-s-max-and-matlab-s-max#answer_124534

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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);