  We apply a Fourier spectral numerical method to 3D incompressible MHD
turbulence with a magnetic Prandtl number $Pr \geq 1$. We examine the processes
by which an initially weak, large-scale seed magnetic field and an initially
weak, small-scale, impulse-like seed magnetic field are amplified. We find that
in both cases the magnetic energy spectrum grows at all scales. The growth
rates at different amplification stages are analyzed. For a large-scale seed
magnetic field, the magnetic energy density grows as $\sim t^2$ for the first
few turbulence eddy turnover times, followed by a dynamic growth stage, where
nonlinear interactions between different scales of the turbulence contribute to
an exponential growth rate that is largely determined by the turbulence eddy
turnover time. For a seed magnetic field that is initially set up at a small
scale in the turbulence, during the kinematic development stage, the growth
rate of magnetic energy is $\propto 1/\tau_{max}$, where $\tau_{max}$ is the
eddy turnover time of the smallest eddies of the turbulence. The kinematic
growth stage is followed by a dynamic growth stage, where nonlinearity plays
important role. During such dynamic growth stage, the growth rate of total
magnetic energy is determined by both the magnetic energy amplification within
the turbulence inertial range and that within the turbulence dissipation range.
