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.Applications I: Switches - Application circuits showing how transistors are used as electronically controlled switches. Operation Modes - An overview of the four possible operating modes of a transistor.Extending the Water Analogy - Going back to the water analogy to explain how a transistor acts like a valve.Symbols, Pins, and Construction - Explaining the differences between the transistor's three pins. 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. This won't be a problem so long as you are aware of the difference, and refer to the datasheet of the part you are using when wiring up the circuit.In small, discrete quantities, transistors can be used to create simple electronic switches, digital logic, and signal amplifying circuits. Some equivalent transistors have the opposite pinout (E,B,C) and some are B,C,E. The 2N3904 comes in a TO-92 case with leads in the order C,B,E. So just look for a substitute that has H FE of at least 100 at an I C of around 10mA, and you should be OK.įinally, before settling on a particular part you might want to check its Packaging - case style, lead designations etc. In your circuit higher gain is better, but 100 is plenty enough. Any substitute that is within or close to that range should be fine (assuming the circuit was designed to accommodate such a wide variation). The manufacturer may sort them and label similar units with the same part number, but they don't throw away any that aren't identical because that would make the 'perfect' remainders too expensive.Īt 10mA Collector current the 2N3904's current gain could be anywhere between 100 and 300. The diffusion process that creates its elements is not precise, so every transistor that is produced has different characteristics. Why so many numbers? The reason relates to how transistors are made. Instead you are given a range with minimum, maximum, and perhaps 'typical' values for different currents and voltages. But confusingly, the datasheet doesn't provide a single number. That leaves DC current gain as the essential parameter that must be duplicated. So you don't have to worry about high frequencies, AC signals, or switching speed, and you have plenty of voltage available so saturation voltage is not critical. Your circuit compares a slowly changing DC voltage to an adjustable reference voltage, and switches on one of two LEDs. Depending on the application, some may be more important than others. Your circuit is only powered with 9V, and the current passing through each transistor is limited to less than 20mA, so any replacement that can handle eg. The 2N3904 is rated for a maximum of 40V between Collector and Emitter, and 200mA Collector current. The perfect replacement would have the same or higher ratings, but they could be lower provided that your circuit doesn't come close to them. These must never be exceeded, or the device will fail. Top of the list is Absolute Maximum Ratings. The chances of of finding an identical substitute are slim, so you will have to decide which numbers are important, and how closely they must match. But beyond that the datasheet may provide a lot of parameters. Obviously the replacement must be of the same type (bipolar NPN) and preferably designed for the same applications (general purpose amplifier). First, find the datasheet of the device you want to replace.
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