BC857BMTF

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Bipolar junction transistors (BJT) are the most ubiquitous type of transistor employed today, and their use has become so commonplace it is hard to imagine any area of electronics that does not include them. This article will focus on one specific type of BJT, the BC857BMTF, and discuss its application field and working principle.

The BC857BMTF is a PNP type of bipolar junction transistor, registering very low Collector-Emitter saturation voltage (Vce(sat)) of 0.2V Min at a collector current of 0.2A and an operating temperature range of -55 to +125C. This type of BJT is suitable for applications requiring switching or amplifying functions, particularly in low-voltage, low-power, low-noise and miniature electronics. These features make the BC857BMTF an ideal choice for a myriad of applications, including contactless switching, small-signal circuits, and many more.

The BC857BMTF operates through an electron flow from the emitter to the collector, with the base controlling its movement. When the base voltage is increased with respect to the emitter voltage, the current flow is increased. When the base voltage is decreased, current flow will decrease in output. The amount of current flowing through the emitter is proportional to the input voltage to the base of the transistor; when the two levels are equal, they are said to be in ‘saturation mode’. This type of BJT also exhibits a positive transconductance behaviour, with an input current (base current) of 1uA initially providing a coefficient of gm of 25mS.

The BC857BMTF can also be used for a variety of amplifier applications. A common configuration for a basic amplifier is the Common-Emitter, which consists of a resistor connecting the base and collector of the transistor, with a small signal applied at the base and a supply voltage at the collector. The resistor in the circuit acts to set the bias voltage at the base, allowing the transistor to operate in the linear region and maintain a constant voltage gain; the supply voltage at the collector helps to determine the output current and power for the transistor.

The current flow in a single BJT is far too low for high-frequency applications, and thus the BC857BMTF is better suited for low-frequency signals, such as those associated with logic circuits. The current gain of a single BJT is also relatively low, with the BC857BMTF exhibiting a hFE (current gain) ranging from 200 to 500. However, BJTs can be connected in a ‘Darlington Pair’ configuration, in which two BJTs are wired in parallel and serve to amplify the signal provided to the base of the first transistor. This configuration allows for very high current gain, along with high input impedance and low output resistance, making it ideal for robust signal amplification.

The BC857BMTF is an excellent choice for applications that require a low-voltage, low-power and low-noise BJT transistor. Its low input bias current coupled with its moderate current gain make it a reliable transistor for applications such as contactless switching, small-signal circuits and many more. By wiring two BC857BMTF in a Darlington pair configuration, these transistors can also provide robust signal amplification, which can be beneficial in wide-ranging applications.

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