Plasma dispersion modulators (PDMs), such as carrier injection modulators and carrier depletion modulators, are widely used for high speed phase modulation in silicon photonic circuits, but they suffer from spurious intensity modulation. This can be a problem in coherent communication systems that make use of complex multi-level quadrature modulation formats, as well as analog applications such as microwave photonics. In this article, a method to achieve pure phase modulation using PDMs is proposed based on a configurable modulator circuit. The configurable modulator is implemented as a Mach-Zehnder interferometer with a PDM and tunable couplers (TCs). The spurious intensity modulation of the phase modulated lightwave can be compensated by tuning the coupling ratios of the TCs and the phase delay between the two arms of the MZI. Simulation results show that for a depletion modulator, the 1.26 dB spurious intensity modulation can be suppressed down to 0.023 dB within a phase range of 0.4, and for injection modulator, the 1.27 dB spurious intensity modulation can be suppressed down to 0.07 dB within a phase range of 0.76.