  We consider a class of nonlinear stochastic differential equations, giving
the power-law behavior of the power spectral density in any desirably wide
range of frequency. Such equations were obtained starting from the point
process models of 1/f^b noise. In this article the power-law behavior of
spectrum is derived directly from the stochastic differential equations,
without using the point process models. The analysis reveals that the power
spectrum may be represented as a sum of the Lorentzian spectra. Such a
derivation provides additional justification of equations, expands the class of
equations generating 1/f^b noise, and provides further insights into the origin
of 1/f^b noise.
