2n3819 bias problem.

Hi,

A while back I built a FET based guitar pre-amp using the 2N3819 device. It's a very simple single stage common-source amplifier powered by a 9v battery, 4k7 load (actually a pot) and 470R source resistor to set bias. Sounds great, etc - so far so good!

My problem is in trying to build more of them. I can't seem to find 2N3819's that bias the same. The 470R source resistor has to be increased considerably to get Vd to be 1/2 supply voltage. That in turn eats into the headroom to a point where the design isn't viable.

Anyone know where I can obtain 3819's that will bias like the first one I tried? So far I have 2 devices that work, and 35 that are unusable. Both the ones that work are Vishay Siliconix, but I have ten that don't. None of the 25 Fairchilds I've tried work. (Work = work in this application).

Thanks in advance!

Steve

I get about 4 fets per 100 that will work the way I want for microphones.

IMO the best way to build stuff with fets is to buy then in large amounts(>100) and pretest them.

Try a J201... Small Bear sells them in small quantites and so does Mouser. I find they are more consistent that many other jfets. The 2SK117 is a good one as well.

regards, Jack

> where I can obtain 3819's that will bias like the first one I tried?

2N3819 is a very loosely-specified part. Vgs(off) could be as high as 8V or as low as... hmmmm, they don't even give a minimum Vgs(off)! Idss could be 2mA or 20mA. You need only about 1mA, which any 2N3819 will pass, but high-Idss and high Vgs(off) parts could take over 6V to cut them back to 1mA, leaving nearly nothing left from a 9V battery.

In short, you may need much-more than 8V supply voltage to set an operating point for any 2N3819.

I think the real use of a part like this is a high-current amplifier with no DC resistance in the load: a tuned radio amplifier. In that application, you generally start with a LOT of current for best weak-signal gain and noise, then apply AVC grid-bias to turn it down for typical stronger signals.

Try 2N5484: it is more the size you need for 1mA current at 0.5 to 2.0V of bias, and you are likely to find more workable units per baggie. This is selected from Process 50, the workhorse JFET wafer, so should be very common.

EDIT: Jack has a good point: J201/J202 is a smaller Process 52 part aimed at low-current audio. J201 maxes out at 1mA so may not be best for 5K load on a 9V battery, J202 may be a better bet here. If you play with various JFET projects, it might be wise to get a baggie of each, J201 and J202. Mouser has them at about $0.40 each, about $8 for a bag of 25. (A dime each if you buy 5,000....)

Cheers for the replies.

No sign of the 2N5484 or J202 in any of the catalogues I have here (I should explain that I'm in the UK, which is a bit like having one arm tied behind your back). They do list the J201 though, but I wouldn't want to run that at max current.

Looks like another of life's necessities that I'll have to have shipped from thousands of miles away. :roll:

Thanks again,

Steve

:guinness:

> No sign of the 2N5484 or J202 ...I'm in the UK

Go to http://uk.farnell.com/ Farnell InOne, search for "J202", check the box "Include Newark InOne Direct Ship..."

I wound up at this monster URL, which may not work for you.

After wading past J201-numbered PC-toys down to Transistors, JFET, I got:

Order Code 4475215
Part No. J202 VISHAY SILICONIX
In Stock £0.24

Can't complain about the price. (Except you need almost 100 to make a minimum order.)

> another of life's necessities that I'll have to have shipped from thousands of miles away.

All these parts are made in the furthest (and cheapest) corners of the earth. They sure don't make them in Newark NJ USA (which at one time was a vacuum-tube hub). (Anyway, I think Newark Electronics is in Chicago now, and they don't make JFETs there either.)

What is Newark InOne Direct Ship?
Newark InOne Direct Ship is exactly what it says. You can now order products on this site from our sister company, Newark InOne, and have them shipped directly to you from the USA, within 3 days.

So you pay with your local money, taxes are handled, and you might have it in about the same time it would take to find a moment to pop into the local parts-shop.

> They do list the J201 though, but I wouldn't want to run that at max current.

No harm will happen, it will just sit there saturated. Change the load resistor to 10K, maybe half of J201 will work well enough for jazz.

> I thought they were just a run of the mil current source.

They are. But the third leg makes them a voltage-variable current source. And that's an Amplifier.

Steve's design needs a device from nearer the LOW-voltage end of the run of the mill, not the high-current ample-voltage end that is popular in RF work. Process 52 is small, good only for low current, what we want. The lowest end of P52 is sold as J201; Steve needs a little more so J202 is more his size, but unfortunately not popular enough to stock locally. (You KNOW there is someone like you with a 2,000-bag of J202 in his county, but where?)

PRR and CJ - Thanks again for the replies.

I obviously didn't look hard enough in the Farnell catalogue. :roll: (Edit: Ah, It's from Newark - that explains it)

IMO, there's nothing like a single FET pre-amp for driving the front end of valve guitar amplifiers. Kind of like having an extra 1/2 an ECC83 in there.

-Steve