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Original Message (ID '322171') By Bill Simpson:
In[1]:= sys=s/(1000+s); ampl=Abs[sys/.{s->2*Pi*f*I}] Out[2]= 2*Pi*Abs[f/(1000 + (2*I)*f*Pi)] In[3]:= 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)] What do you want the derivative of Abs of a complex value to be? What is Abs'[3/I] for example? When you can decide that then perhaps a function can be found that will provide you Real values for a plot.