The document discusses the three common configurations of bipolar transistors: 1. Common base configuration has voltage gain but no current gain. The input is between base and emitter, and output is between base and collector. 2. Common emitter configuration has both current and voltage gain. It is the most widely used configuration in transistor amplifiers. The input is between base and emitter, and output is between collector and emitter. 3. Common collector configuration has current gain but no voltage gain. It is useful for impedance matching. The input is at the base, and output is across the emitter load resistor. The collector is common to both input and output.