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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 12th Jan 2017, 10:28 am   #21
'LIVEWIRE?'
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Default Re: 2N3819 Mosfet

There also are (or were) TO-18 metal can versions of the 2N3819. I bought some several years ago, from where I forget, and don't recall the pinouts of those.
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Old 12th Jan 2017, 11:44 am   #22
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Default Re: 2N3819 Mosfet

I built the Mini-Mod back in June 2015 and used a BF256 in preference to the 2N3819. I connected the FET as per the data sheet, and it wouldn't work. I discovered that despite what the data sheet stated, the actual pin outs differed. I posted the difficulties in post 85 at the thread below, along with the PCB that I built, and posted the successful outcome at post No 95:

https://www.vintage-radio.net/forum/...ad.php?t=98221

(The mini mod still works a treat!)
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Old 12th Jan 2017, 12:06 pm   #23
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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.

The fact that many JFETs will work with source/drain either way was mentioned by G6Tanuki and kalee20 and seems to have been confirmed by a few others since. They are symmetrical devices, although it might not be quite correct to say it would work with all JFET devices or 'with equal efficiency' although the difference should not be noticable in practice. It will definitely not work for IGFETs or MOSFETs.

I see Phil G4SPZ has now confirmed that he followed the Farnell layout and this worked fine so hopefully it will work for your project as well.
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Old 12th Jan 2017, 1:14 pm   #24
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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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Old 12th Jan 2017, 4:01 pm   #25
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Default Re: 2N3819 Junction gate field-effect transistor (JFET)

Thanks guys, I have learnt something I never knew about about JFET transistors.
Assembly is proceeding with the Minimod on veroboard, I will update my progress on Phil G4SPZ's thread, https://www.vintage-radio.net/forum/...ad.php?t=98221

thanks again
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Old 12th Jan 2017, 4:06 pm   #26
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Default Re: 2n3819 Mosfet

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Originally Posted by Philips210 View Post
As G6Tanuki points out, the drain and source are interchangeable in many circuits.
Not usually on Jfets, especially depletion as they are not symmetrical. Just like Bipolar transistors DO work reversing C & E, but only then less than 5V HT (low Veb zener action) and about 1/10th gain. The CB is avalanche mode breakdown rather than EB zener)

Not on IG-VFETs, because a parasitic Drain Source diode. Sometimes called MOSFETs.

I think only reliably on Enhancement mode IG-MOS-FETs without the inherent diode on Drais/Source. Hence they are used as transmission gate switches.

I'd use a J310, better and cheaper and easier to get than 2N3819.
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Old 12th Jan 2017, 4:17 pm   #27
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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.

The project I am working on is a Minimod TX, Phil G4SPZ has been sent me a photo of one he made so I will be fitting the JFET in the same way.
The positive supply is fed to the JFET by the red and green wire.
My first MiniMods used the 2N3819 FET, and yes, the pinouts are confusing! The gate is in the centre and as has been stated, many FETs are more or less functionally symmetrical.

I changed the FET in later versions for a BF256 as it had a tighter spec - I had some 2N3819s that just were not very good. But beware - the BF256 gate is at one end of the three leads not in the centre.

Ian
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Old 12th Jan 2017, 4:33 pm   #28
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Default Re: 2N3819 Junction gate field-effect transistor (JFET)

Quote:
Originally Posted by crackle View Post
... I will update my progress on Phil G4SPZ's thread, https://www.vintage-radio.net/forum/...ad.php?t=98221
You'll have a job, Mike, that thread's closed! Perhaps you could start a new thread in the 'homebrew equipment' section?
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Old 12th Jan 2017, 5:32 pm   #29
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Default Re: 2N3819 Junction gate field-effect transistor (JFET)

You also have the option of sending a request to a moderator to have the thread opened again.
I am in the very early stages of making a MOSFET based HT bench power supply.
If I build it the output would be 0V to 250V like a low voltage transistor type.
I did not know that JFETs sometimes work both ways.
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Old 12th Jan 2017, 7:05 pm   #30
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Default Re: 2N3819 Junction gate field-effect transistor (JFET)

I will start a new thread then,

I tested the 2N3819 in my little Chinese transistor, capacitor, everything-else tester and no matter which way round it identified it as a FET, 1,2,3 sgd so long as g was in the centre.

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Old 12th Jan 2017, 8:12 pm   #31
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Default Re: 2N3819 Junction gate field-effect transistor (JFET)

As everyone is pointing at, the Drain and Source are interchangeable.

I used to wonder over this and eventually got the definitive way to tell from the late and great Robert (Bob) Pease of National Semiconductor.
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Old 12th Jan 2017, 8:21 pm   #32
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Default Re: 2N3819 Junction gate field-effect transistor (JFET)

Quote:
Originally Posted by Mooly View Post
I used to wonder over this and eventually got the definitive way to tell from the late and great Robert (Bob) Pease of National Semiconductor.
Wonderful

The most famous Bob Pease quote: "My favourite programming language is solder"
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Old 12th Jan 2017, 8:55 pm   #33
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Default Re: 2N3819 Junction gate field-effect transistor (JFET)

Quote:
Originally Posted by mhennessy View Post
The most famous Bob Pease quote: "My favourite programming language is solder"
Are you sure? I thought that was Steve Ciarcia (as in 'Circuit Cellar' and 'Circuit Cellar Ink')
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Old 12th Jan 2017, 10:13 pm   #34
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Default Re: 2N3819 Junction gate field-effect transistor (JFET)

They both said it. Who said it first - and whether one copied the other - is open to debate.
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Old 12th Jan 2017, 10:20 pm   #35
Phil G4SPZ
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Default Re: 2N3819 Junction gate field-effect transistor (JFET)

To prove a point, I have just tested a 2N3819 on my Comark type 185 FET tester. I set up the transistor at +10 volts Vds and -1.5 volts Vgs, and I obtained exactly the same values of Ids and gm with the device connected either way round.
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Old 12th Jan 2017, 10:48 pm   #36
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Default Re: 2N3819 Junction gate field-effect transistor (JFET)

There are soe designs of HF transceiver where the same IF and IF filters and diode-ring mixers are used in one direction for receiving, and in the other direction for transmitting. These need 'bilateral' amplifiers which can be switched to amplify in one direction or the other. One design used for this, and which avoids lots of RF switches is to use a carefully chosen symmetric J-Fet and to switch the DC power feed from one end to the other.

David
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