A unipolar power supply is a power supply that can deliver power in one quadrant. Meaning that the supply can only provide positive voltage and current and thus positive power. A bipolar power supply on the other hand, can provide both negative and positive power. Bipolar supplies can also operate in either two or four quadrants. A two-quadrant bipolar power supply can provide positive voltage and source and sink current while a four-quadrant bipolar power supply can provide positive and negative voltage as well as source and sink current.