How to compute the derivative of the neural network?

Hi, Once you have trained a neural network, is it possible to obtain a derivative of it? I have a neural network "net" in a structure. I would like to know if there is a routine that will provide the derivatives of net (derivative of its outputs with respect to its inputs).   It is probably not difficult, for a feedforward model, there is just matrix multiplications and sigmoid functions, but it would be nice to have a routine that will do that directly on "net".