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Old 12th Jan 2017, 2:14 pm   #24
kalee20
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Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
Default Re: 2N3819 Mosfet

Quote:
Originally Posted by crackle View Post
So is that a fact the 2N3819 JFET can be used either way round, with equal efficiency, the only pin which needs to be specific is the gate.
Wow that does seem strange.
Basically a JFET is just a length of reasonably heavily doped n-type material (the channel) with connections at each end. So it will pass current either way!

Then, it is surrounded by a sleeve of p-type material (the gate). If the gate is biased negatively with respect to the channel, no current will flow to the gate, because it's a reverse biased PN junction. But more significantly, the electrons in the n-type channel near the p-type sleeve get repelled away from it, thus reducing the cross-sectional area in the channel where electrons are to be found. This reduces its ability to conduct from one end to the other, so you can vary the channel current by changing the gate voltage.

Although basically symmetric, some FETs have one end of the channel designated the source, the other designated the drain, with the cross-section not constant and the gate positioned nearer one end than the other. It's so that capacitance can be reduced between the gate and the 'drain' end, helping in circuit design (we all know how output-input feedback is a pain).

The 2N3819 is a cheap FET, with rather a wide spread of characteristics from one sample to the next. It's the sort of thing that makes you grateful that bipolar transistors exist! But aside from its unpredictability, it can be made to perform well.
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