Bipolar/4-quadrant power supplies
Bipolar power supplies are power supplies which support 4-quadrant operation, which means that they can source and sink both positive and negative volatges and currents.
The voltage polarity of general power supplies is fixed to a single polarity, thus they are called unipolar power supplies. If voltage is applied to a resistive load from a unipolar power supply, current that is proportional to the voltage flows in a fixed direction. The current flows from the output voltage's positive terminal to the output voltage's negative terminal.
Unlike with resistive loads, when a power supply that alternates voltage polarity based on time is applied to an inductive or capacitive load, the current phase becomes lagging or leading to the voltage phase. If this occurs, there are four possible combinations of the voltage and current polarities. This operation is shown on a graph of voltage (vertical axis) versus current (horizontal axis) and bipolar supplies are ones that can operate in all four quadrants of this Cartesian plane.