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A C945 transistor is a type of negative-positive-negative (NPN) bipolar junction transistor. Typically, circuits where a low-current, high-speed transistor is required will employ a transistor such as the C945 transistor. Circuits such as a small-signal amplifier or a high-speed switching circuit might employ one or more C945 transistors. A C945 transistor can be used in several types of electronic circuits, but it is best suited for use in low-power applications.
Bipolar junction transistors contain three semiconductor regions: the collector, the base and the emitter. An NPN bipolar junction transistor — such as the C945 — contains a base region that is doped with positive, or P-type, semiconductor material, along with collector and emitter regions that are doped with negative, or N-type, semiconductor material. This configuration allows the C945 transistor to conduct electric current between the collector and emitter regions when voltage is applied to the transistor’s base region.
Some switching devices, such as electromechanical relays, might be too slow to turn a circuit on or off. An electronic circuit containing a C145 transistor can be used in a switch circuit that requires high-speed switching. A typical relay might take five milliseconds to engage and 10 milliseconds to disengage, but a C145 transistor can switch on or off 150 million times per second.
A C145 transistor also can be used in an amplifier circuit. Amplifier circuits use a transistor and direct current (DC) voltage to increase the power level of an input alternative current (AC) signal. For an amplifier that is based on an NPN transistor, the AC signal is passed through the base region on the transistor. A voltage differential between the collector and emitter regions on the transistor add power to the AC signal, and the output AC signal is obtained by connecting the output device to the collector region on the transistor.
Each bipolar junction transistor contains several ratings that serve as a guideline for how the transistor is to be used. A C145 transistor is best used in low-power applications. The maximum voltage that can be applied from the collector region to the emitter region on a C145 transistor is 50 volts; otherwise, the transistor will overheat and fail. The maximum voltage that can be applied from the base region to the emitter region is 5 volts. The total power that can be applied safely to a C145 transistor is 0.4 watts.
Many NPN bipolar junction transistors are packaged in a TO-92 plastic case. Many of these transistors contain three discrete leads that go to the transistor’s semiconductor regions. Often, one of the outer leads provides an interface to the transistor’s collector region, the center lead provides an interface to the base region, and the remaining outer lead provides an interface to the emitter region. It is important for the transistor schematic to be checked to verify the electrical lead wire assignments.