Question: NonlinearFit Parameter Specification

Hey, I'm using NonlinearFit to find parameterranges in theoretical equations for physical data. however some of the parameter values are being returned as negative values and I need to specify parameterranges but am having trouble doing this. 

For example in the equation below I would like to specify that alpha and ro should take on positive values, as otherwise with the curret data set I am using ro is assigned a negative value.

g1 := NonlinearFit(E(F, alpha, ro, .31, 165, e), X, Y, F)

Also in this equation I would like to be able to specify a range for b, eg b=1..20.

g2 := NonlinearFit(E(F, b, 165, e), X, Y, F)

Can someone please give me an example of how to do the above. Your help will be much appreciated. Tanks