Author 
Comment/Response 
Bill Simpson

02/06/13 8:25pm
In Response To 'Re: Re: derivative of amplitudefrequency chara...'  I apologize for my last message. The text I added to the bottom of my reply to try to explain the problem that I was seeing was badly worded and confusing.
Here is part of your original code, if I have not made any more mistakes:
In[1]:= sys=s/(1000+s);
ampl=Abs[sys/.{s>2*Pi*f*I}];
D[ampl,f]
Out[3]= 2*Pi*(((2*I)*f*Pi)/(1000 + (2*I)*f*Pi)^2 + (1000 + (2*I)*f*Pi)^(1))*Abs'[f/(1000 + 2*I)*f*Pi)]
That Out[3] is, I believe, one of the things you want to plot.
Do you get the same result if you do this?
If so then observe the first part of that does not appear to be a Real valued function of f and the second part of that is the derivative of the absolute value function which my Mathematica does not know how to further evaluate.
It is that second part which I tried to (badly) question in my previous message to you.
So I believe the reason you are not seeing anything from your Plot is that you do not have a Real valued function of f. Check each of those to see if you can confirm that for yourself.
If you can confirm that then we need to figure out what needs to be changed to get the unknown Real valued function.
I hope this is a better explanation.
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