![]() I connected the collector to a 10-VDC power source through a 1-kΩ resistor and used a 1-MΩ resistor between the transistor’s base and the 10-V power supply. I used Labcenter Electronics’s Proteus VSM in my example, but you can use any SPICE tool (e.g., Linear Technology’s free LTSpice) or an online version (e.g., CircuitLab, PartSim, etc.).įigure 3 shows a basic circuit built around a BC238B. Simulating a transistor is straightforward using a linear circuit simulator (e.g., SPICE), even if you prefer to wire it. Even in a single class, the gain dispersion from part to part could be huge. The Fairchild Semiconductor BC238 exists in three gain classes, indicated by an A, B, or C suffix. It is not strictly constant, but still close to 0.6 V, as explained. This means this specific transistor’s current gain is 50 mA divided by 200 ♚, which is 250. As soon as the VCE voltage is above a couple of volts, the current circulating through the collector is nearly constant, around 50 mA. Look for an example at the curve for IB = 200 ♚. Each curve corresponds to a given base current (IB). Figure 2a shows you the relationship between the voltage between the collector and the emitter (VCE) and the current through the collector (IC). Figure 2 shows a reproduction of the BC238B’s key characteristics from its datasheet. The examples in this article are based on the old faithful Fairchild Semiconductor BC238B transistor, but you could use any common NPN transistor (e.g., the ubiquitous 2N2222, the 2N3904, or the BC847 if you prefer surface-mount packages). Wikipedia also provides a good BJT summary.) (Search for “Ebers-Moll model” online if you are interested in the details. Of course, this is an approximate explanation, as the transistor’s physics are a little more complex, but is enough for my example. The current through the collector will always be HFE times higher.įor example, if you have a transistor with a gain of 100 and inject 1 mA in the base, then 100 mA will flow through the collector. In this mode, a given current will circulate through the base. You will not be able to increase the base voltage significantly above 0.6 V and the device will start to be current-controlled. If this voltage is increased to the threshold, then the transistor becomes active. Here’s how it works: If the voltage applied between the base and the emitter is lower than this threshold, then the transistor is blocked and no current circulates through the collector. Second, the voltage between the base and the emitter is stable and close to 0.6 V for more devices, as with any bipolar diode: Their ratio is the transistor current gain, which is indicated in the transistor’s datasheet and often noted ßF or HFE: Two basic equations dictate its behavior.įirst, the current circulating through the collector is roughly proportional to the current applied on the base. This NPN bipolar junction transistor is wired in the common-emitter configuration, meaning its emitter is grounded. With this setting, the emitter is grounded. Let’s focus on the basic “common emitter” circuit (see Figure 1). Due to their internal semiconductor structure, the currents circulating through each of these terminals, as well as the voltages between them, are all linked together. BJT transistors have three terminals: collector (C), emitter (E), and base (B). However, by reversing the power supply rails, everything would be applicable to its PNP cousin. For simplicity, I will focus on the NPN version. ![]() The BJT comes in two flavors, NPN and PNP. Brattain and John Bardeen, who were on William Shockley’s team. In 1947, it was discovered at the Bell Laboratories by Walter H. Moreover, understanding what’s going on with simple parts can’t hurt, and transistors can even be fun! That’s why this month I will provide a refresher on how to use a one-cent bipolar junction transistor (BJT) to build an amplifier. What a shame! A transistor can be a more adequate and cost-effective solution than an IC in many projects. Many electronics engineers are fluent with complex systems-such as microcontrollers, embedded OSes, or FPGAs-but seem to have more difficulties with single transistors. Going back to the basics is never a bad idea.
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