  The magnetic compound Ba_2CuGe_2O_7 has recently been shown to be an
essentially two-dimensional spiral antiferromagnet that exhibits an
incommensurate-to-commensurate phase transition when a magnetic field applied
along the c-axis exceeds a certain critical value H_c. The T=0 dynamics is
described here in terms of a continuum field theory in the form of a nonlinear
sigma model. We are thus in a position to carry out a complete calculation of
the low-energy magnon spectrum for any strength of the applied field throughout
the phase transition. In particular, our spin-wave analysis reveals
field-induced instabilities at two distinct critical fields H_1 and H_2 such
that H_1 < H_c < H_2. Hence we predict the existence of an intermediate phase
whose detailed nature is also studied to some extent in the present paper.
