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The 2N222 transistor is a common negative-positive-negative (NPN) bipolar junction transistor (BJT) that finds use in many different kinds of electronic equipment. It is used for both analog signal amplification and switching applications. The functioning parts of the 2N2222 transistor are enclosed in what is known as a TO-18 package, which resembles a small metal can. The broad range of uses for the 2N2222 transistor, and its small size, make it — and its variants — the most widely used transistors in electronics.
The functioning portion of the 2N2222 transistor is a NPN BJT construct. The 2N2222 transistor is made of either germanium or silicon that has been saturated with either a positively or negatively changed material in a process called “doping.” The 2N2222 has a positively charged section sandwiched between two negatively charged sections. The resulting two connections between the three sections are where the 2N2222 derives the name “bipolar junction transistor.” The materials used are arranged in the order of negative, positive, then negative, so the device is also said to be a NPN transistor.
The 2N2222 has three wire leads used to solder it to circuit boards: the collector, the emitter, and the base. When an electronic signal is present at the transistor’s collector, applying a signal to the transistor’s base will cause a signal to emit from the device’s emitter. In this way, the 2N2222 is often used to switch signals on and off.
The switching abilities of the 2N2222 transistor also make it useful as a simple “and” gate. When used in this capacity, the transistor will only send a signal when two separate signals are present: one at its collector and one at its base. This allows the 2N2222 to be used to automatically control signal flow in a circuit depending on what signals are, or are not, present.
In amplification applications, the 2N2222 receives an analog signal, such as an audio signal, through its collector and a separate signal is applied to its base. The output at the transistor’s emitter will then be identical to the collector signal with the exception that it increases in power by an amount proportional to the signal applied to its base. Additionally, varying the signal applied to the base will vary the amplification of the signal leaving the emitter.
Operational characteristics make the 2N2222 a low- to medium-current (up to 600 milliamps), low-power (up to 625 milliwatts), medium-voltage (up to 40 volts) device. Though these parameters may seem to limit the 2N2222’s usefulness, the 2N2222 is perfect for a host of signal manipulation and handling applications prior to high-power amplification. 2N2222 transistors are also used to condition signals before and after application to more advanced digital devices.
While the 2N2222 was the first of its kind, it has spawned a number of variants collectively called “2N2222 type” transistors, because they all share functional-construction and operational characteristics identical to the original 2N2222 transistor. Chief among these variants is the P2N2222 transistor, which is enclosed in a small black TO-92 package made of either epoxy or plastic. The combination of the large number of uses for the 2N2222 and the cost-effective TO-92 package has made the P2N2222 the least expensive and most used transistor in electronics.