Transistor

There are three ways to connect a bipolar junction transistor into a working circuit, depending upon which of the three transistor elements is chosen as the common reference for the other two elements. These variations, called common base, common emitter, and common collector, produce different circuit actions each with unique characteristics. An n-p-n transistor configured as a common-emitter amplifier, where both the base and the collector circuits are referenced to the emitter, is normally connected with a positive voltage on the collector, as referenced to the emitter. The collector-base diode and the base-emitter diode appear to be in series, connected back-to-back. The collector-base diode is reverse biased so that almost no current will flow unless the base-emitter diode is forward biased. The very small current in the collector circuit under these conditions is because the p-type material in the base is starved for the n-type majority carriers that the collector circuit requires if it is to conduct a significant current. When the base-emitter junction is forward biased, the carriers needed for current in the collector circuit find their way into the collector.

The base-emitter diode in the transistor offers a very low resistance to current flow when it is forward biased. It is therefore very easy to cause current in the transistor's input circuit. Since the base region is made very thin, most of the majority carriers that flow from the emitter will be caught by the strong electric field in the collector base junction before they can exit through the base connection. It takes only a small amount of power to cause current in the transistor's forward-biased base-emitter input circuit yet almost all this easily forced input current appears in the collector circuit. A low-powered signal becomes a higher-powered signal when the input current caused by a low voltage appears almost undiminished in the collector circuit, but at a higher voltage.