What Is a C945 Transistor?

David Sandoval

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.

Transistors are typically identified by a printed code, which often includes one or two letters followed by a series of numbers.
Transistors are typically identified by a printed code, which often includes one or two letters followed by a series of numbers.

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.

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Discussion Comments

I actually remember using a few transistors on what's called a breadboard back in high school. It was the beginning of the science class, and the teacher was trying to introduce us to electricity and electrical circuits. Most times things didn't light up or power on properly but working with those transistors was fun.

Observing the world around us today it would seem as if we are now on the upswing to build larger everything. There was a time when it seemed manufacturers were determined to cram as many components on as small a PCB board as possible. Now things are headed in the opposite direction smart-phones are getting larger, tablets and laptops along with monitors and flat screen televisions are increasing in size I think you would either need to increase the amount or the size of the transistors in some of these devices in order for it them work properly without overheating.

I'm certain there are transistors that are larger than what you can find in the local electronics outlet store. These would most likely be designed and built for specific and unique purposes. I don't know much about the various sizes of transistors but there are larger ones that exist.

I remember learning about these when I was in high school. I was always fascinated by the concept of pushing power through such miniscule components and watching the devices they were in come to life. I wondered to myself just how big you could make one of these? Most of the ones I have seen have been small radio component sized devices. I would assume the bigger the transistor the larger the power needed to make it function properly.

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