Temperature dependent soft mode part of the complex dielectric constant has been measured as a function of bias electric field both in the smectic C* and smectic A* phases for a ferroelectric liquid crystal (FLC) mixture. From the experimental data, soft mode relaxation frequency and dielectric strength have been determined both in smectic C* and smectic A* phases. In smectic B* phase, relaxation frequency and inverse of dielectric strength of the FLC mixture does not follow the Curie-Weiss law. The critical exponent for the studied FLC mixture is 1.43. Dielectric results shows a dependence of soft mode dielectric strength and relaxation frequency with bias field near the transition temperature in smectic A* phase to an extent of 1 degrees C above the transition temperature and above 1 degrees C bias field does not have any effect on relaxation frequency and dielectric strength It is observed that relaxation frequency increases with increasing bias held while the corresponding dielectric strength decreases.