Non-Phool jellybean audio-frequency JFET

[Ecnerwal]

Trying to pick a JFET that's still being made with reasonable low noise
characteristics for simple audio preamp type duties. No golden-ear BS,
just things that can be measured. Mic preamps, instrument pickup
preamps, a FET to have in the junkbox for unknown things yet to be
cobbled in the small-signal audio range. Cheap is also good. Jeorg cheap
would be even better ;-) Old-fashioned packaging would be nice, but its
unlikely these days, I think.

Mouser has one item that comes up with Audio JFET (that's not a jfet
input something else...) Audio FET gets a few more, but most are class D
power devices.

Toshiba 2SK880 in "irritatingly tiny" package. Par for the course these
days and I have adapted to soldering irritatingly tiny if I have no
other choice in packaging. 43 cents for 1, $29.50 for 100 Looks to be 5
years old judging by the datasheet date.

One that is mentioned in some older web circuits that's still marginally
available (in the 150% larger SOT23 only) is the J201, which seems to
have somewhat worse noise numbers. 23 cents for 1, $21.90 for 100, and
all of its relatives in other packages are already obsolete, so it may
not be long for this world, either?

Digi-key's search is as usual near useless (or it and I search
differently), and Newark comes up with a bunch of class-D power fets
that probably won't like non-switching use (If I have even a vaguely
correct recollection of what "class D audio amp" means. Looks like I do
per 5 seconds of checking my memory)

There are of course lots of RF parts that have no specs below 100Khz, or
1 Mhz, or 1 Ghz, depending on part. Perhaps some of them work fine for
audio. Anyone care to clue me in?

--
Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.

[Phil Hobbs]

A few BF862s in parallel, for choice. About 0.8 nV 1-Hz noise in the
flatband, 1/f corner around 1 kHz.

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

[George Herold]

On Aug 20, 12:33 pm, Ecnerwal

OPA2134? ~$2 at the 100 quant. 8nV/rtHz, (only 8MHz)

George H.

[George Herold]

On Aug 20, 12:33 pm, Ecnerwal
<MyNameForw...@ReplaceWithMyVices.Com.invalid> wrote:

Hey, I was leafing through electronic design at lunch, and thought of
you when I saw an add for Jfets from linear systems.
(linearsystems.com)

George H.

[Phil Hobbs]

BF862s, really. They're the cat's pajamas.

[amdx]

On 8/22/2012 1:45 PM, Phil Hobbs wrote:
> George Herold wrote:
>>
>> On Aug 20, 12:33 pm, Ecnerwal
>> <MyNameForw...@ReplaceWithMyVices.Com.invalid> wrote:
>>> Trying to pick a JFET that's still being made with reasonable low noise
>>> characteristics for simple audio preamp type duties. No golden-ear BS,
>>> just things that can be measured. Mic preamps, instrument pickup
>>> preamps, a FET to have in the junkbox for unknown things yet to be
>>> cobbled in the small-signal audio range. Cheap is also good. Jeorg cheap
>>> would be even better ;-) Old-fashioned packaging would be nice, but its
>>> unlikely these days, I think.


>

> BF862s, really. They're the cat's pajamas.
>
> Cheers
>
> Phil Hobbs
>

Just because I was curious I searched BF862 preamp and found a
high performance phono stage that uses 8 parallel BF862s.

http://www.synaesthesia.ca/LNschematics.html

3/4 downpage

I'm clueless as to whether it is a good circuit.

Mikek

[Phil Hobbs]

BF862s parallel just fine. But for audio, even just one is better than
good enough. There's no need for subnanovolt noise in audio.

[George Herold]

On Aug 22, 2:45 pm, Phil Hobbs

> hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -
>
> - Show quoted text -

Geesh Phil, I don't know. Mostly I can't tell the difference between
a Jfet and a J stroke.

(They've got BF862's at newark for ~$0.30/100)
... I might have a use for a 0.8nV Fet. Where's the 1/f knee?
At the moment it's a ~10kHz resonace, but I could move that up, some.

George H.

[George Herold]

On Aug 22, 4:39 pm, Phil Hobbs

> hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -
>
> - Show quoted text -

You 'pay' 10pF of input C for each one.

(According to the 'one' npx spec sheet I looked at.)

George H.

[Phil Hobbs]

> You 'pay' 10pF of input C for each one.
>
> (According to the 'one' npx spec sheet I looked at.)
>
> George H.

Right, but this is audio after all. I use them in boatloads to make
photodiode bootstraps that work up to ~20 MHz. You put a good-quality
current source in the sources, which gets rid of Cgs pretty well, and
then bootstrap the drains to get rid of Cdg and Cds. You still get the
voltage noise differentiated by Cdiode+Cin, but since they're so quiet,
that's still a big win.

I'm experimenting with using BF862/pHEMT combos with diplexers to get
low 1/f noise and extreme bandwidth.

[Phil Hobbs]

> Geesh Phil, I don't know. Mostly I can't tell the difference between
> a Jfet and a J stroke.
>
> (They've got BF862's at newark for ~$0.30/100)
> ... I might have a use for a 0.8nV Fet. Where's the 1/f knee?
> At the moment it's a ~10kHz resonace, but I could move that up, some.
>
> George H.

It's about 1 kHz. They're firmly in the flatband at 10 kHz.

[amdx]

On 8/22/2012 1:45 PM, Phil Hobbs wrote:

Hey Phil,
Do you have a compliment to the BF862?
Mikek

[amdx]

Ok, ok, Hello BF862, you've got some really nice parameters there.
Do you have a complement for the BF862?
Mikek

[Phil Hobbs]

Don't I wish. Try a BF862 in an inverted cascode with some nice quiet
PNP transistor, e.g. a 2N5087 at low frequency or a BFT92 at high frequency.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

[amdx]

>>>
>>> BF862s, really. They're the cat's pajamas.
>>>
>>> Cheers
>>>
>>> Phil Hobbs
>>>
>>
>> Hey Phil,
>> Do you have a compliment to the BF862?
>> Mikek
>
> Don't I wish. Try a BF862 in an inverted cascode with some nice quiet
> PNP transistor, e.g. a 2N5087 at low frequency or a BFT92 at high
> frequency.
>
> Cheers
>
> Phil Hobbs
>

Ya, most of this is over my head, I was just thinking about
replacements for an AM radio antenna preamp circuit I know of.
Is this a physics problem building a P type to match an N type?
Thanks, Mikek

[Ecnerwal]

They fit the mold of what I was thinking might be the case - an "RF"
part with application elsewhere. I had actually found Phil's earlier
postings praising them (and verified that they were still available)
before I posted, but was not at all clear after looking at a datasheet
that cut off frequency specs at 0.1 Mhz on the low end if they'd work
reasonably for audio, given that they were being touted for RF in car
radios and a lot of what Phil does goes up there a ways.

Quite a price variance between digikey/newark/mouser, with mouser
winning strongly (27 cents for 1, 23.3 for 100.)

Given a lack of directly comparable graphs I have a hard time telling if
the slightly more expensive (43/29.9) and smaller (smaller not being an
advantage for me as a one-off tinkerer) 2SK880 has a _slightly_ lower
corner frequency or not really, but I suspect it's not enough of a
difference to actually matter (not going phoolish), given that I'm
mostly looking for a reasonable part to have on hand and play with in
the same way I have a pile of 3904/6, LM833, etc.

Now to see how I do with Larkin's x-acto/copper foil prototyping
technique, since I can't see getting boards made for every iteration of
goofing around I want to do, and these things are seriously small...

[Phil Hobbs]

I bought a reel of BF862s a year or two back for $650, or about 22 cents
each.

Using SOT23s in protos isn't too hard. You can mount them easily on the
pad-per-hole style of perf board, or get some prototyping adapters, e.g.
the Bellin Systems ones.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

[Phil Hobbs]

Yes. The hole mobility in silicon is low, which makes the
transconductance low, which makes P-channel devices noisier.

BJTs don't have the same issue since the base is so narrow and the
transconductance is very high, independent of device polarity. (PNPs
used to be slightly quieter than NPNs, but not any more.)

[George Herold]

On Aug 23, 9:12 am, Phil Hobbs

Great, that's perfect. A colleague was chattering about active
damping (as was used to damp torsional fibers back in the day.) And I
mentioned that we could try active damping to kill some of the johnson
noise in a high Q RCL circuit. A nice low noise jfet might be
perfect. But I need to think about it some more.

George H.

>
> Cheers
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal Consultant
> ElectroOptical Innovations LLC
> Optics, Electro-optics, Photonics, Analog Electronics
>
> 160 North State Road #203
> Briarcliff Manor NY 10510
> 845-480-2058
>

[Phil Allison]

"Ecnerwal"

>
> Trying to pick a JFET that's still being made with reasonable low noise
> characteristics for simple audio preamp type duties.

** JFETs have many desirable properties and one HUGE drawback.

The sample to sample parameter spread is massive - so much so that it is
normal to select devices for a given circuit so that bias / operating point
conditions will be met.

If you need diff pairs with low input offsets - then be prepared to waste
a lot of FETS.

FET input op-amps and matched FETs on a chip are the way to go.



.... Phil

[Phil Hobbs]

That's all too true of every JFET that I know about except one: the
BF862. Have a look at the datasheet--they're magic. I'd never use a
JFET in anything if it weren't for these ones. Their transconductance
is very high, so the action is all over in about 400 mV. They're very
predictable for a JFET, comparable to a pHEMT, and almost as good as a
BJT. You just parallel them up and away you go.

[Phil Hobbs]

Phil Hobbs wrote:
>
> Phil Allison wrote:
> >
> > "Ecnerwal"
> > >
> > > Trying to pick a JFET that's still being made with reasonable low noise
> > > characteristics for simple audio preamp type duties.
> >
> > ** JFETs have many desirable properties and one HUGE drawback.
> >
> > The sample to sample parameter spread is massive - so much so that it is
> > normal to select devices for a given circuit so that bias / operating point
> > conditions will be met.
> >
> > If you need diff pairs with low input offsets - then be prepared to waste
> > a lot of FETS.
> >
> > FET input op-amps and matched FETs on a chip are the way to go.
> >
> > .... Phil
>
> That's all too true of every JFET that I know about except one: the
> BF862. Have a look at the datasheet--they're magic. I'd never use a
> JFET in anything if it weren't for these ones. Their transconductance
> is very high, so the action is all over in about 400 mV. They're very
> predictable for a JFET, comparable to a pHEMT, and almost as good as a
> BJT. You just parallel them up and away you go.
>

One other thing: Since they're so quiet, and not _that_ well matched, I
very often use a BF862 follower driving the inverting input of a very
low noise bipolar op amp such as an ADA4898. Use an adjustable current
sink to bias the FET's source, and use a FET op amp in a very slow
feedback loop, holding V_GS at zero. (This is often called "snooping
the summing junction". Do it via a 10k-1M resistor to keep from loading
the SJ, and keep the snooper's loop bandwidth low enough that you don't
care about the big resistor's noise.)

That gets you an excellent FET input amp for inverting applications: <
100 pA input current, ~1.2 nV/sqrt(Hz) noise, ~100 MHz GBW. With a few
AC fiddles, e.g. bootstrapping various things, this makes a really
brilliant TIA among other things.

You can do the same sort of thing for noninverting use, but you have to
be a bit careful about the large signal performance of the current sink
and the snooper.

[George Herold]

On Aug 24, 9:42 am, Phil Hobbs

> hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -
>
> - Show quoted text -

Very cool thanks Phil.
I think I followed about 1/2 of that. (You make me feel like a cave
man rubbing two resistors together at times.)
I was thinking about using the jfets as a differential pair in front
of a 'nice' opamp. (a B. Pease circuit fragment.)
For a simple resistor only circuit, I've recently been turned on to
this artifical resistor circuit by R.L. Forward.
(US patent 4176331, or J. Appl. Phys. (53) 3365, 1982
Three resistors and an opamp. Just my speed! (I'm still working on
the noise analysis.)

("Dragons Egg" (sci fi.) by R.L. Forward is a fun read.)

George H.

[Phil Hobbs]

Didn't mean to be mysterious about it. The main op amp runs with its
noninverting input grounded, so its feedback will hold the BF862's
source at ground. The remaining problem is to make sure that the gate
of the BF862 is also at ground, for which you need a current sink in its
source that's adjusted to exactly I_DSS.

The current sink should be a BJT with a couple of volts drop across its
emitter resistor (which gets rid of its shot noise pretty well). The
snooper op amp is connected as a slow integrator, with its inverting
input connected to the gate of the BF862 through a sufficiently large
resistor, and its noninverting input grounded. Its output controls the
current source. (Make sure you can get any current from about 8 to 25
mA, and watch out that you don't crank up the current high enough to
forward-bias the GS junction.)

The bad news is that you get the Johnson noise of the big resistor, but
it goes away for frequencies more than ~10 times the loop BW. Another
RC bypassing the base of the BJT to the negative supply helps with high
frequency noise and PSRR.

That way the BF862 always runs at exactly I_DSS, and you avoid the
offset, drift, and extra noise caused by using a BF862 diff pair.

> I was thinking about using the jfets as a differential pair in front
> of a 'nice' opamp. (a B. Pease circuit fragment.)

The main problem with that, as with all composite amps, is frequency
compensating it without getting all sorts of whoop-de-doos at late times
in the step response. (Putting a pole-zero pair inside a feedback loop
doesn't get rid of it entirely--it replaces it with two closely spaced
pairs, and the error shows up as ~1%-ish ripples in the step response.)

> For a simple resistor only circuit, I've recently been turned on to
> this artifical resistor circuit by R.L. Forward.
> (US patent 4176331, or J. Appl. Phys. (53) 3365, 1982
> Three resistors and an opamp. Just my speed! (I'm still working on
> the noise analysis.)
>
> ("Dragons Egg" (sci fi.) by R.L. Forward is a fun read.)

Thanks. I'm an old Forward fan, ever since coming across some of his
science writing when I was about 12. I've read Dragonfly, Rocheworld,
and a few others. Good medicine. He also wrote a really great paper
about using interferometers to detect gravity waves in about 1972.

[Phil Hobbs]

BF862. About 0.8 nV noise, 1/f corner about 1 kHz. They parallel very
nicely (we had a thread on that a couple of months back). When
parallelled directly, you want to run them at I_DSS, so an op amp servo
to force V_GS to be zero. If you're just using one, run it at about 12 mA.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

[Phil Hobbs]

Weird. This showed up on ES as a new post with no replies, and then
suddenly all this discussion from 2012 appeared.

Ah well, even antique circuit discussion is better than zillions of
posts about graduated cylinders and email clients, which in turn is
better than some of the other stuff we get stuck in sometimes. New
circuits discussion is better still of course.

[Jim Thompson]

You just don't appreciate the finer things in life >:-}

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

[John Larkin]

On Tue, 13 Jan 2015 12:42:22 -0500, Phil Hobbs
<pcdhSpamM...@electrooptical.net> wrote:

What do you have against bread pudding recipes?


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

[Jim Thompson]

Hobbs probably doesn't like grits either ;-)

[Phil Hobbs]

> What do you have against bread pudding recipes?
>
>

Bread pudding is a crime against humanity.

[Phil Hobbs]

Well, on my current ketogenic diet, it's in the same category as cotton
candy. ;)

[John Larkin]

On Tue, 13 Jan 2015 15:13:57 -0500, Phil Hobbs

I suppose I could work up a version that has a lot of meat in it.

--

John Larkin Highland Technology, Inc

picosecond timing laser drivers and controllers

[Jim Thompson]

That would ruin it. Ignore those people who don't relish a good bread
pudding.

[Phil Hobbs]

Or just save the bread, eggs, and sugar for something more like this:

http://tinyurl.com/q8bo96z

[John Larkin]

This is my Custard Bread Pudding. Custard on the bottom, bread and
fruit on top.

https://dl.dropboxusercontent.com/u/53724080/Food/CBP.JPG

Phil doesn't like this sort of thing. Maybe I could make the top layer
bacon. Or just take him to The House Of Prime Rib.

Last night I invented Shrimp and Bacon over fettucine, with a light
garlic cream sauce. Sort of a Shrimp Carbonara. Got good reviews.

I have a really interesting oscillator phaselock problem with wild
numerical things going on, vaguely like the old Tektronix "Random
Sampling" thing, but I can't discuss it in public. Pity.

[Jim Thompson]

On Wed, 14 Jan 2015 08:25:16 -0800, John Larkin

Looks good!

>
>Phil doesn't like this sort of thing. Maybe I could make the top layer
>bacon. Or just take him to The House Of Prime Rib.
>
>Last night I invented Shrimp and Bacon over fettucine, with a light
>garlic cream sauce. Sort of a Shrimp Carbonara. Got good reviews.

I regularly grill shrimp (and scallops) wrapped in bacon.

>
>I have a really interesting oscillator phaselock problem with wild
>numerical things going on, vaguely like the old Tektronix "Random
>Sampling" thing, but I can't discuss it in public. Pity.

Most everything I do I can't talk about, until it's antiquated :-(

[John Fields]

On Wed, 14 Jan 2015 08:25:16 -0800, John Larkin
<jla...@highlandtechnology.com> wrote:

.
.

.

>I have a really interesting oscillator phaselock problem with wild
>numerical things going on, vaguely like the old Tektronix "Random
>Sampling" thing, but I can't discuss it in public. Pity.

---
If you can't discuss it, why on Earth would you even allude to it?

I think we both know the answer to that one...

https://www.youtube.com/watch?v=mQZmCJUSC6g