The slow light effect in semiconductor optical amplifiers has many applications in microwave photonics such as phase shifting. An amplified sinusoidally amplitude modulated lightwave leads to carrier density fluctuations at the modulation frequency. This leads to a change in the effective group index and after photodetection the beat signal (at the modulation frequency) at the output is phase shifted relative to the input beat signal. A potential alternative scheme is to use the detected electrode voltage as the photodetector. The voltage depends on dynamic changes in the conduction-valence band quasi-Fermi level difference. The feasibility of the proposed scheme, using bulk reflective SOA, is shown by using a time-domain model that includes band-structure based calculations of the SOA material properties, a carrier density rate equation, and travelling-wave equations for the amplified signal and spontaneous emission.