Current freely flows from collector to emitter. Saturation - The transistor acts like a short circuit.(When we talk about current flow through a transistor, we usually mean current flowing from collector to emitter of an NPN.) They have four distinct modes of operation, which describe the current flowing through them. Unlike resistors, which enforce a linear relationship between voltage and current, transistors are non-linear devices. There's a lot more to it, but that's a good place to start! Check out the next section for a more detailed explanation of the operation of a transistor. Transistors are special because they can amplify electrical signals, turning a low-power signal into a similar signal of much higher power. We're stretching the analogy to its limits, but this idea carries over to transistors too. The measly amount of force you might put into twisting that knob has the potential to create a force thousands of times stronger. Imagine if, with the slight turn of a valve, you could control the flow rate of the Hoover Dam's flow gates. There's another analogy we can wrench into this. So, in a way, a transistor is like a variable, adjustable resistor. If a valve can finely adjust the width of a pipe, then a transistor can finely adjust the resistance between collector and emitter. Transistors are created by either stacking an n on top of a p on top of an n, or p over n over p.Ī transistor can do the same thing - linearly controlling the current through a circuit at some point between fully off (an open circuit) and fully on (a short circuit).įrom our water analogy, the width of a pipe is similar to the resistance in a circuit. A semiconductor material with extra electrons is called an n-type ( n for negative because electrons have a negative charge) and a material with electrons removed is called a p-type (for positive). Some of those layers have extra electrons added to them (a process called "doping"), and others have electrons removed (doped with "holes" - the absence of electrons). Transistors are built by stacking three different layers of semiconductor material together. Using the diode (or resistance) test function on a multimeter, you can measure across the BE and BC terminals to check for the presence of those "diodes".) Transistor Structure and Operation (This model is useful if you need to test a transistor. There's a whole lot of weird quantum physics level stuff controlling the interactions between the three terminals. Don't base your understanding of a transistor's operation on that model (and definitely don't try to replicate it on a breadboard, it won't work). The diode representation is a good place to start, but it's far from accurate. The diode connecting base to emitter is the important one here it matches the direction of the arrow on the schematic symbol, and shows you which way current is intended to flow through the transistor. By narrowing our focus down - getting a solid understanding of the NPN - it'll be easier to understand the PNP (or MOSFETS, even) by comparing how it differs from the NPN. We'll turn our focus even sharper by limiting our early discussion to the NPN. Digging even deeper into transistor types, there are actually two versions of the BJT: NPN and PNP. In this tutorial we'll focus on the BJT, because it's slightly easier to understand. There are two types of basic transistor out there: bi-polar junction (BJT) and metal-oxide field-effect (MOSFET). Applications II: Amplifiers - More application circuits, this time showing how transistors are used to amplify voltage or current.
TRANSISTOR 2N3904 SERIES
This tutorial is split into a series of sections, covering: We won't dig too deeply into semiconductor physics or equivalent models, but we'll get deep enough into the subject that you'll understand how a transistor can be used as either a switch or amplifier. Covered In This TutorialĪfter reading through this tutorial, we want you to have a broad understanding of how transistors work.
In quantities of thousands, millions, and even billions, transistors are interconnected and embedded into tiny chips to create computer memories, microprocessors, and other complex ICs. In small, discrete quantities, transistors can be used to create simple electronic switches, digital logic, and signal amplifying circuits.
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