We demonstrate that the sensitivity limit of intensity-based surface plasmon resonance (SPR) biosensors can be enhanced when we combine the effects of the phase and amplitude contributions instead of detecting the amplitude variation only. Experimental results indicate that an enhancement factor of as much as 20 times is achievable, yet with no compromise in measurement dynamic range. While existing SPR biosensor systems are predominantly based on the angular scheme, which relies on detecting intensity variations associated with amplitude changes only, the proposed scheme may serve as a direct system upgrade approach for these systems. The new measurement scheme may therefore lead to a strong impact in the design of SPR biosensors.