What Is a BC337 Transistor?

Geisha A. Legazpi

A BC337 transistor is a three-terminal current control device used for general purposes. Based on the semiconductor type used, the BC337 transistor is a negative-positive-negative (NPN) type because the main terminals use a negative (N)-type material. The transistor schematic for an NPN transistor indicates the emitter with an arrow pointing away from the transistor. It is known as a bipolar junction transistor because it uses a positive (P)-type semiconductor material sandwiched between two N-type materials. Semiconductors, such as silicon and germanium, are used in the BC337 transistor to control the flow of electrical charges by allowing a control current to vary the conductivity of the transistor’s main terminals.

Transistors -- such as the BC337 -- may be used in circuits to amplify or switch electrical signals that pass through them.
Transistors -- such as the BC337 -- may be used in circuits to amplify or switch electrical signals that pass through them.

This NPN transistor has a base or the control terminal, an emitter terminal, and a collector terminal. On the base-to-emitter junction, the amount of current flow determines the collector current. The ratio of the collector current to the base current is known as beta. If a transistor has a beta of 100, a current of 1 milliampere (mA) through the base-emitter produces a current of 100 mA in the collector. The collector could be connected to a passive load, which could be a resistor, a tuned circuit, an audio transformer, or a relay’s electromagnetic coil, onto a direct current (DC) power supply.

The collector-to-emitter junction of the BC337 transistor may act as a switch for simple DC switching operations. If a 12-volt (V) relay coil requires 100 mA to be fully energized, the 100 mA may be produced by a 12 volts direct current (VDC) supply in series with the relay coil and the collector-to-emitter junction of the transistor. Given a beta of 100, it takes only 1 mA at the base to produce the 100 mA at the collector. The 1 mA may be produced at the base when a dark condition is shown on a phototransistor. This effect may be used to turn on a light bulb via a relay when the room gets dark.

There are many transistor characteristics to consider when using a BC337 transistor. The collector to emitter voltage should be below the maximum collector voltage, and the current on the collector should not exceed the maximum collector current specified for the device. A transistor array carries several transistors inside one package, a scheme that is very useful for designs that use several of the same transistor. In 8-bit computing, for instance, the bus drive may require eight transistors. A transistor array can help minimize total package count, which can simplify final circuit assembly.

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Discussion Comments


@Mammmood - Yeah, but that amplification can be a double edged sword. You have to guard against too much current going in according to the article otherwise it could damage the transistor and any other components to which it’s attached.

You probably need some kind of check system to guard the base of the transistor and ensure that the voltage is within acceptable parameters.

Perhaps a bunch of resistors in series could do it, I don’t know. It’s been awhile since I messed with this stuff, and when I did, all I had to do was to follow existing schematics rather than come up with my own designs.


I think the silicon transistor is a great all around component for amplifying voltages. The fact that you need only a fractional ampere at the base to produce voltage necessary for tripping a relay is incredible.

I think that’s why transistors like this one are used a lot in audio applications. Amplifiers need to boost audio signals that come in so that they can increase the volume as needed.

Transistors are great for this. They probably have a whole bunch of them assembled in an array; obviously it would take more than one transistor alone to do the trick. It makes me wonder how we got by without these little marvels.

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