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Old 10th Sep 2018, 9:46 am   #9
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Building the 'Minimod' AM modulator-questions!

Just one observation on this circuit...

The J-FETs used have very large - absolutely huge - variation from part to part allowed in their data sheets. The 2N3819's max current (at Vgs=0) can be anywhere between 2 and 20mA and if you bias it for 0.2mA current the gate-source voltage may have to be anywhere between 0.5 and 7.5 volts.

What this means is that not all examples of those FETs will work in the circuit, and you may be unlucky and find one which isn't a go-er. You may have to try ones from a different batch before you get lucky.

So if you're making an early foray into the world of field effect transistors, don't be discouraged if it takes a bit of work to make it go. It isn't something you've done wrong. It is VERY difficult to design sure-fire circuits with JFETs. And simple looking circuits usually aren't the sure-fire ones.

2N3819 is THE original popular JFET. Later part types came in graded bands so the spreads were reduced, a bit like the coloured dot of paint on early bipolar transistors, though some parts didn't get suffixes, the different grades sometimes got different type numbers like 2N4391, 2N4392, 2N4393.

David
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