For the rest of the article, except when specifically addressing this issue, we shall use the positive cases (NPN & N-channel) for all our examples.Īlthough FETs had been demonstrated and patented almost twenty years earlier than BJTs 1 the first practical transistors were bipolar 2. This is critically important, but so obvious that little further discussion is needed on the topic. The most basic question of all when choosing a transistor, though, is not whether it's a BJT or an FET but its polarity - in use is its output terminal positive or negative with respect to its common terminal? If the answer is positive we need an NPN BJT or an N-channel FET, otherwise we need a PNP or a P-channel. There are two basic types of transistor - bipolar junction transistors and field-effect transistors, known respectively as BJTs and FETs. But we do need to know what they do and it may be helpful to know a little about why they behave as they do - so we'll talk, just a little, about transistor structures. There are plenty of textbooks which give a good summary of the basics and there are innumerable other books and articles on both basic principles and detailed studies of particular issues. We shall not discuss the physics of transistors here. How do we make a good choice of a transistor without wasting time on unnecessary detail?
Which do we choose - and why?įor many applications there is no need to choose a particular transistor - we should just use the first reasonably suitable one that comes to hand. There are tens of thousands, possibly hundreds of thousands, of different types of discrete transistor and there are almost always a few places in a system where a discrete transistor is necessary. The correct question is not “Where do I get that specific device?” but “What other, easily obtained, devices will work in this application?” One of the common questions the author and his colleagues in the Applications Department are asked is “The application note for XXXX calls for a 3N14159 transistor - where can I get one?” Research reveals that the 3N14159 has been obsolete for years - or is only obtainable (in minimum orders of 1,000,000 pieces) with a lead time of 21 months from a factory in Timbuktu.
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