The free electrons move through the collector region, into the external circuit,and then return into the emitter region along with the base current,as indicated. The emitter current is slightly greater than the collector current because of the small base current that splits off from the total current injected into the base region from the emitter.

The directions of the currents in an npn transistor and its schematic symbol are shown in above Fig these for a pnp transistor are shown in above Fig. Notice that the arrow on the emitter inside the transistor symbols points in the direction of conventional current.

These diagrams show that the emitter current ( I E) is the sum of the collector current ( I C) and the base current ( I B), expressed as follow:Īs mentioned before, I B is very small compared to I E or I C. The capital-letter subscripts indicate dc values. Two important parameters, β DC (dc current gain) and α DC are introduced and used to analyze a BJT circuit. Also,transistor characteristic curves are covered,and you will learn how a BJT’s operation can be determined from these curves. Finally, maximum ratings of a BJT are discussed. When a transistor is connected to de bias voltages for both npn and pnp types, V BB forward-biases the base-emitter junction, and V CC reverse biases the base-collector junction. Although in this chapter we are using separate battery symbols to re[present the bias voltages, in practice the voltages are often derived from a single de power supply.For example, V CC is normally taken directly from the power supply output and V BB (which is smaller) can be produced with a voltage divider. The DC current gain of a transistor is the ratio of the de collector current ( I C) to the de base current ( I B) and is designed de beta (β DC ).

Bipolar square wave