A 2N3904 transistor is a bipolar negative-positive-negative (NPN) transistor, which means it is usually applicable to negative ground circuitry. It may be used for audio signals as well as for medium-speed switching applications. This small transistor is the counterpart of the 2N3906 transistor, which is a positive-negative-positive (PNP) transistor. By injecting a small base current to a 2N3904 transistor, a larger collector current can be produced.
This transistor has three terminals called the emitter, the base, and the collector. The emitter and the collector are the main terminals of the 2N3904 transistor. Depending on the circuit configuration, the load or load equivalent may be connected either to the emitter or to the collector.
One of the various parameters of the 2N3904 transistor is known as the beta, or current gain, which is the ratio of the collector current to the base current. For a current gain of 100, a change of 0.001 ampere (A) in base current results in a change of 0.1 A on the collector. This suggests how the 2N3904 transistor can become an amplifier. A small change in the base current leads to a hundred times change in the collector current, which can translate into a voltage or power change. By designing transistor stages in cascade, it is possible to build amplifiers, switches, and oscillators for various applications.
Biasing refers to the idle currents on the terminals of the transistor. As a general rule, the 2N3904 transistor requires a forward bias on the base to emitter, meaning that there is a positive potential on the base with reference to the emitter. It should be noted that the base is a positive (P)-type material, while the emitter is a negative (N)-type material. The amount of forward bias has to be controlled depending on the specific application. Too much forward bias will usually cause excessive collector current, which usually leads to saturation.
When there is a reverse bias on the base-to-emitter junction, the collector current is usually near zero. This leads to a cutoff, or the condition that happens when the collector current is almost zero. In switching and radio frequency applications, operations near cutoff are used for shutting down load current at the collector. For radio frequency applications, operating near cutoff makes it possible to “pulse” a special circuit known as a tank circuit, which resonates much like the way a pendulum bob swings when it is given a nudge.