sot jfet

J

John Larkin

Guest
Hi,

I need a p-channel jfet in a sot-23. Who makes stuff like that?

John
 
John Larkin wrote:

Hi,

I need a p-channel jfet in a sot-23. Who makes stuff like that?

John

Try ON, Zetex and Philips for starters. Then let us know if you find more.
Click to expand...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
In article <10uiklrn927up8e@corp.supernews.com>,
Tim Wescott <tim@wescottnospamdesign.com> wrote:
John Larkin wrote:

Hi,

I need a p-channel jfet in a sot-23. Who makes stuff like that?

John

Try ON, Zetex and Philips for starters. Then let us know if you find more.
Click to expand...
IIRC Central Semi does too.

--
--
kensmith@rahul.net forging knowledge
 
John Larkin wrote:
(snip)
So the choices are either a small p-ch mosfet, if I can find one with
low enough capacitance, or one of those "digital transistors" with a
built-in base resistor. Or a really slow PNP, if anybody still makes
one.
Click to expand...
Like this?
http://rocky.digikey.com/WebLib/Fairchild/Web%20Data/2N5086_87_MMBT5086_87.pdf

So listen up, kids: ALL EMITTER FOLLOWERS NEED BASE RESISTORS unless I
give you permission otherwise.

John
Click to expand...

--
John Popelish
 
On Sat, 15 Jan 2005 23:28:46 -0500, John Popelish <jpopelish@rica.net>
wrote:

John Larkin wrote:
(snip)
So the choices are either a small p-ch mosfet, if I can find one with
low enough capacitance, or one of those "digital transistors" with a
built-in base resistor. Or a really slow PNP, if anybody still makes
one.

Like this?
http://rocky.digikey.com/WebLib/Fairchild/Web%20Data/2N5086_87_MMBT5086_87.pdf
Click to expand...
Thanks, but no. The graphs show Ft as around 300 MHz in the 5-10 mA
range, almost the same as the BCX71. It will probably oscillate too.
This is for a $6 billion laser, so I can't take chances.

I had thought that the output impedance of the opamp would be enough
base degen to prevent oscillation, but I guess the wire bonds, esd
diodes, and the output transistors can form a high-enough-Q circuit to
sustain oscillation. Obviously so, in retrospect.

If I ask the production folks to lift the base lead and stick an 0402
resistor in there, they'll be after me with torches and pitchforks.

John
 
John Larkin <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote:

Thanks, guys, but false alarm.

I've got this box full of programmable time delays. Each one is a PNP
transistor current source driving a capacitor, reset by a gaasfet,
making about a 60 ns linear ramp into a comparator with a 16-bit DAC
on the other side, so I get 0-60 ns delay in about 1 ps steps. The
current source is a BCX71 PNP inside an opamp feedback loop (fb from
source resistor off +15 rail), conventional stuff. Like an idiot, I
left out a series base resistor so the PNP likes to oscillate at
around 100 MHz, causing a bunch of jitter in the delays. These are
Click to expand...
The feedback loop itself is stable?

--
Reply to nico@nctdevpuntnl (punt=.)
Bedrijven en winkels vindt U op www.adresboekje.nl
 
In article <ijsju0lup2mi1c1jdbapun39fec23grp3m@4ax.com>,
John Larkin <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote:
[...]
I had thought that the output impedance of the opamp would be enough
base degen to prevent oscillation,
Click to expand...
The output impedance of an op-amp is alway such that it makes oscillation
more likely. They tend to look like parallel LC circuits. The L part is
because as the frequency increases, the servo loop gain is dropping
causing a rising impedance. The C part is the various stray capacitances.



If I ask the production folks to lift the base lead and stick an 0402
resistor in there, they'll be after me with torches and pitchforks.
Click to expand...
Can you deQ the emitter side of things or slip a bead in the collector's
path?

--
--
kensmith@rahul.net forging knowledge
 
On Sun, 16 Jan 2005 00:33:39 -0500, John Popelish <jpopelish@rica.net>
wrote:

John Larkin wrote:

On Sat, 15 Jan 2005 23:28:46 -0500, John Popelish <jpopelish@rica.net
wrote:

John Larkin wrote:
(snip)
So the choices are either a small p-ch mosfet, if I can find one with
low enough capacitance, or one of those "digital transistors" with a
built-in base resistor. Or a really slow PNP, if anybody still makes
one.

Like this?
http://rocky.digikey.com/WebLib/Fairchild/Web%20Data/2N5086_87_MMBT5086_87.pdf


Thanks, but no. The graphs show Ft as around 300 MHz in the 5-10 mA
range, almost the same as the BCX71. It will probably oscillate too.
This is for a $6 billion laser, so I can't take chances.

I had thought that the output impedance of the opamp would be enough
base degen to prevent oscillation, but I guess the wire bonds, esd
diodes, and the output transistors can form a high-enough-Q circuit to
sustain oscillation. Obviously so, in retrospect.

If I ask the production folks to lift the base lead and stick an 0402
resistor in there, they'll be after me with torches and pitchforks.

John

It occurs to me that the transistor may be feeding the oscillations
because it is too slow to maintain the current steady, but feeds back
collector voltage to the base. Have you tried a much faster
transistor lower gain, lower collector to base capacitance, but higher
fT)?

Something like:
http://rocky.digikey.com/WebLib/Panasonic/Web%20data/2SA1738.pdf
Click to expand...

I used the BCX71 because in the past the RF-types (NEC, or the Moto
parts when they still made them) were *really* bad about oscillating.

My first experience with this problem was a long time ago, when I
needed a TTL powerup reset signal. I did this...


+---------+--------- +5
| |
r |
| c
+--------b 2N2219
| e
| |
c |
| +----------ttl gate------> reset sig
gnd |
r
|
gnd


which never made a high to the ttl gate, it was oscillating so hard at
100 MHz or so. A small base resistor, 33 or 47 maybe, generally fixes
it.

So most small-signal bipolars are hazardous when operating as emitter
followers when the base sees a low-Z source. As the transistor gets
faster, things seem to get worse, with the traces and wirebonds to the
base becoming microwave resonators.

Considering how often this seems to happen, at least to me, it's
surprising how seldom it's mentioned. The AoE example (p 300) is
similar, but blames the oscillation here on impedance in the collector
circuit.

John
 
On Sun, 16 Jan 2005 09:15:18 GMT, nico@puntnl.niks (Nico Coesel)
wrote:

John Larkin <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote:

Thanks, guys, but false alarm.

I've got this box full of programmable time delays. Each one is a PNP
transistor current source driving a capacitor, reset by a gaasfet,
making about a 60 ns linear ramp into a comparator with a 16-bit DAC
on the other side, so I get 0-60 ns delay in about 1 ps steps. The
current source is a BCX71 PNP inside an opamp feedback loop (fb from
source resistor off +15 rail), conventional stuff. Like an idiot, I
left out a series base resistor so the PNP likes to oscillate at
around 100 MHz, causing a bunch of jitter in the delays. These are

The feedback loop itself is stable?
Click to expand...

Yes. The opamp Ft is only about 3 MHz, and the oscillation is around
100. It's definitely local to the transistor.

John
 
I read in sci.electronics.design that John Popelish <jpopelish@rica.net>
wrote (in <41EAB889.1733CDE3@rica.net>) about 'sot jfet', on Sun, 16 Jan
2005:
I would love ot have seen the layout that this circuit was made with
that oscillated so vigorously. I have been designing and laying out
circuits that contained various uses for emitter followers, and have had
only a few bad experiences with them. And in every case, I was able to
cure the oscillation with a better layout. Maybe I have just been very
lucky.

Inductance in the emitter lead creates a negative impedance at the base.
Click to expand...
I expect that's why the thing oscillates. Inductance in the collector
lead could possibly also cause trouble, but the oscillation is often
less persistent.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
On Sun, 16 Jan 2005 10:16:24 -0800, John Larkin wrote:

My first experience with this problem was a long time ago, when I
needed a TTL powerup reset signal. I did this...


+---------+--------- +5
| |
r |
| c
+--------b 2N2219
| e
| |
c |
| +----------ttl gate------> reset sig
gnd |
r
|
gnd


which never made a high to the ttl gate, it was oscillating so hard at
100 MHz or so. A small base resistor, 33 or 47 maybe, generally fixes
it.
Click to expand...
That's not the only problem with this circuit. You're not supposed to
drive TTL with a current source and pulldown. You're supposed to use an
NPN sink, and a pullup. They also make reset generator chips, but it's a
little late for that now. I'd maybe add an NPN where you have your "ttl",
and some positive feedback to get a good snap-action.

Cheers!
Rich
 
John Larkin wrote:
(snip)

Considering how often this seems to happen, at least to me, it's
surprising how seldom it's mentioned. The AoE example (p 300) is
similar, but blames the oscillation here on impedance in the
collector
circuit.
Click to expand...
To be fair, that example had a d'Arsonval meter movement in the
collector lead.

--
John Popelish
 
On Thu, 20 Jan 2005 13:05:16 +1300, Terry Given <my_name@ieee.org>
wrote:


In a fit of inspired stupidity, or CEO purchased 1,500 pcbs of the new,
completely untested (although heavily SPICEd) circuit, despite our loud,
written protests that it *probably wont work* (ah, the joys of smt
prototyping). He actually went to the purchasing dept and made them
change the order - we had ordered 10. Of course they didnt work, and the
pcb's had to be scrapped. Next management meeting, R&D were criticised
by said CEO for wasting $10,000 on pcb's.
Click to expand...
$10000 / 1500 = $6.66 per board, pretty cheap. He got off easy.

Why didn't somebody stand up and call him a stupid fathead? Better
yet, why didn't everybody call him a stupid fathead?

John
 
On Wed, 19 Jan 2005 18:03:39 -0500, Boris Mohar
<borism_-void-_@sympatico.ca> wrote:

On Sat, 15 Jan 2005 08:52:38 -0800, John Larkin
jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote:

Hi,

I need a p-channel jfet in a sot-23. Who makes stuff like that?

John

http://www.vishay.com/fets-small-signal/SSFsgpchjP/



Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca
Click to expand...
Alas, a jfet would be ideal in a closed-loop current source: low Cout,
zero base-current error, unlikely to oscillate. But the pinouts are
wrong, so I can't drop one into the existing boards.

John
 
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