> In article <b549465b-526d-4394-bf23-84157436d2cd@googlegroups.com>,
> George Herold <gherold@teachspin.com> wrote:
>> So I was searching for a model of a jfet.
>> Figure 1 here,
>> http://www.linearsystems.com/lsdata/others/LIS_White_Paper_Consider_Discrete_JFET.pdf
>>
>> Is the gate and 'back gate' connected together?
>
> From https://www.nxp.com/docs/en/application-note/AN211A.pdf I see
>
> "The substrate, which functions as Gate 2 of Figure 1, is
> of relatively low resistivity material to maximize gain. For the
> same purpose, Gate 1 is of very low resistivity material,
> allowing the depletion region to spread mostly into the n-type
> channel. In most cases the gates are internally connected
> together. A tetrode device can be realized by not making
> this internal connection."
>
> I have a few surplus JFETs from Linear Integrated Systems which do
> have an active fourth lead - I believe it's the substrate (gate 2).
> If I recall correctly one can either tie the substrate to the gate, or
> to the source, or to a constant voltage which is more negative than
> the source (in the case of an N-JFET).
>
>
>
>
>
Having substrate access sounds like it could be useful - shame the jfets
I use (J177, J113, J107, BF256 etc) only have three pins :(
piglet
Reply by Dave Platt●June 28, 20192019-06-28
In article <b549465b-526d-4394-bf23-84157436d2cd@googlegroups.com>,
George Herold <gherold@teachspin.com> wrote:
From https://www.nxp.com/docs/en/application-note/AN211A.pdf I see
"The substrate, which functions as Gate 2 of Figure 1, is
of relatively low resistivity material to maximize gain. For the
same purpose, Gate 1 is of very low resistivity material,
allowing the depletion region to spread mostly into the n-type
channel. In most cases the gates are internally connected
together. A tetrode device can be realized by not making
this internal connection."
I have a few surplus JFETs from Linear Integrated Systems which do
have an active fourth lead - I believe it's the substrate (gate 2).
If I recall correctly one can either tie the substrate to the gate, or
to the source, or to a constant voltage which is more negative than
the source (in the case of an N-JFET).
Reply by John Larkin●June 28, 20192019-06-28
On 28 Jun 2019 08:02:08 -0700, Winfield Hill <winfieldhill@yahoo.com>
wrote:
>George Herold wrote...
>>
>> Winfield Hill wrote:
>>> George Herold wrote...
>>>>
>>>> We sell a PD with a 10 position switch, 1,3.3,10...
>>>> It's OK, when I tried to make it faster I found
>>>> the switch added ~5 pF of C. Otherwise I'm not
>>>> sure what's wrong with a gain switch.
>>>
>>> Well, as I said, the Eckel scheme, with outputs
>>> running simultaneously, insures fast, accurate
>>> digitizing, especially with noisy signals. But
>>> John's scheme looks very practical for separating
>>> a high Rf first stage from a lower Rf second stage.
>>> The first stage would not suffer from any extra
>>> capacitances to slow it down. You could safely
>>> add switches to the lower-gain stage to get lots
>>> of ranges. I might use 10x for 1st to 2nd stage,
>>> to insure speed, and do the 3.3x bit later.
>>
>> Thanks Win, I like the trick. I'm wondering what
>> the application is, where having to switch ranges
>> (with a switch) is a problem.
>
> A switch, with wiring to the panel, will add 1 to
> 5pF across the feedback resistor. We often make
> fast TIAs, with high f_T opamps, that make use of
> the intrinsic 0.1pF capacitance of many resistors.
> We go further, and trick the resistor into having
> even less capacitance, see Figure 8.80-C. These
> applications could use John's scheme to separate
> the highest-gain stage from the rest.
You can sometimes switch things around with low-capacitance diodes or
phemts or even multiplexers, so rotary switch capacitance and wiring
aren't in the circuit. The switch is cold, just DC.
These Fujitsu relays are dynamite.
https://www.dropbox.com/s/14mt8y78cc5ng79/Relays.jpg?raw=1https://www.dropbox.com/s/se162xpw86hpmzs/DSC06884.JPG?raw=1
DPDT, fraction of a pF, fraction of an ohm, good to a few GHz. But
don't water wash them.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
Reply by George Herold●June 28, 20192019-06-28
On Friday, June 28, 2019 at 8:11:36 AM UTC-4, George Herold wrote:
> On Thursday, June 27, 2019 at 4:16:06 AM UTC-4, piglet wrote:
> > On 27/06/2019 2:06 am, George Herold wrote:
> > > On Wednesday, June 26, 2019 at 5:22:34 PM UTC-4, piglet wrote:
> > >> On 26/06/2019 5:48 pm, Winfield Hill wrote:
> > >>> Here's a TIA circuit published in 2012, in RSI,
> > >>> by Yale physicist, Stephen Eckel. “A high dynamic
> > >>> range, linear response transimpedance amplifier.”
> > >>>
> > >>> It's easy to implement, and super useful. The TIA
> > >>> has multiple ranges, each with its own output, but
> > >>> multiple ranges are active at once; there's no loss
> > >>> of data as would happen switching range resistors.
> > >>>
> > >>> Stephen and his co-authors found a simple, clever
> > >>> trick to prevent input TIA opamp saturation, using
> > >>> JFETs to successively short series-placed higher-
> > >>> value range resistors for strong input currents.
> > >>>
> > >>> They suggest a three-stage implementation, with a
> > >>> 300:1 ratio for each, but you can use many stages
> > >>> (each one takes few extra parts), to obtain high
> > >>> accuracy with a say 12-bit ADC. Also, a high
> > >>> input-opamp Vos needn't degrade the dynamic range.
> > >>>
> > >>> DropBox has a draft of our x-Chapters write-up:
> > >>> https://www.dropbox.com/s/fs4edz7dqgwswoj/4x.3.7_Eckel_TIA.pdf?dl=0
> > >>>
> > >>> I think you can download Stephen's RSI article here:
> > >>> https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&ved=2ahUKEwjk6KvVwYfjAhVlRN8KHaMfD48QFjAHegQICRAC&url=http%3A%2F%2Fwww.bmo.physik.uni-muenchen.de%2F~riedle%2FElektronik_I%2FKW103%2F2012_Eckel%2CSushkov_9-decade_RSI.pdf&usg=AOvVaw3g9i6-pWAwuuZlZLvlFArd
> > >>>
> > >>>
> > >>
> > >>
> > >> Marvellous - thank you!
> > >>
> > >> Jfets are nice because when fully depleted off there is no parasitic s-d
> > >> diode that curses gumdrop mosfets. But gumdrop mosfets are much cheaper
> > >> than jfets these days. I wonder if one could use series connected
> > >> source-source 2N7002 and s-s BSS84 to replace those jfets? Eight mosfets
> > >> could work out cheaper than four jfets? While gate leakage should be a
> > >> non-issue I don't know how s-d channel leakage compares?
> > >>
> > >> piglet
> > >
> > > I was going to ask what about source-source fets, and found this,
> > > https://electronics.stackexchange.com/questions/79028/understanding-two-mosfet-with-sources-connected
> > >
> > > Is that right... just looking at pics not reading comments/ words.
> > >
> > > George H.
> > >
> >
> > Yes, here is sketch showing how might be done using mosfets to replace
> > jfets:
> >
> > <https://www.dropbox.com/s/bidm47zj29moq6k/SteppedTIA.jpg?dl=0>
> >
> > piglet
>
> Thanks piglet. I didn't know that Jfet's have no S-D body diode.
> (Why the heck is that? so much I don't know!)
>
> George H.
>
> Winfield Hill wrote:
>> George Herold wrote...
>>>
>>> We sell a PD with a 10 position switch, 1,3.3,10...
>>> It's OK, when I tried to make it faster I found
>>> the switch added ~5 pF of C. Otherwise I'm not
>>> sure what's wrong with a gain switch.
>>
>> Well, as I said, the Eckel scheme, with outputs
>> running simultaneously, insures fast, accurate
>> digitizing, especially with noisy signals. But
>> John's scheme looks very practical for separating
>> a high Rf first stage from a lower Rf second stage.
>> The first stage would not suffer from any extra
>> capacitances to slow it down. You could safely
>> add switches to the lower-gain stage to get lots
>> of ranges. I might use 10x for 1st to 2nd stage,
>> to insure speed, and do the 3.3x bit later.
>
> Thanks Win, I like the trick. I'm wondering what
> the application is, where having to switch ranges
> (with a switch) is a problem.
A switch, with wiring to the panel, will add 1 to
5pF across the feedback resistor. We often make
fast TIAs, with high f_T opamps, that make use of
the intrinsic 0.1pF capacitance of many resistors.
We go further, and trick the resistor into having
even less capacitance, see Figure 8.80-C. These
applications could use John's scheme to separate
the highest-gain stage from the rest.
--
Thanks,
- Win
Reply by George Herold●June 28, 20192019-06-28
On Thursday, June 27, 2019 at 8:20:01 AM UTC-4, Winfield Hill wrote:
> George Herold wrote...
> >
> > We sell a PD with a 10 position switch, 1,3.3,10...
> > It's OK, when I tried to make it faster I found
> > the switch added ~5 pF of C. Otherwise I'm not
> > sure what's wrong with a gain switch.
>
> Well, as I said, the Eckel scheme, with outputs
> running simultaneously, insures fast, accurate
> digitizing, especially with noisy signals. But
> John's scheme looks very practical for separating
> a high Rf first stage from a lower Rf second stage.
> The first stage would not suffer from any extra
> capacitances to slow it down. You could safely
> add switches to the lower-gain stage to get lots
> of ranges. I might use 10x for 1st to 2nd stage,
> to insure speed, and do the 3.3x bit later.
>
>
> --
> Thanks,
> - Win
Thanks Win, I like the trick. I'm wondering what the application is, where
having to switch ranges (with a switch) is a problem. Our diode laser
can use a large range of measurement currents (mA to ~10 nA) But not
at the same time.
George H.
Reply by George Herold●June 28, 20192019-06-28
On Thursday, June 27, 2019 at 4:16:06 AM UTC-4, piglet wrote:
> On 27/06/2019 2:06 am, George Herold wrote:
> > On Wednesday, June 26, 2019 at 5:22:34 PM UTC-4, piglet wrote:
> >> On 26/06/2019 5:48 pm, Winfield Hill wrote:
> >>> Here's a TIA circuit published in 2012, in RSI,
> >>> by Yale physicist, Stephen Eckel. “A high dynamic
> >>> range, linear response transimpedance amplifier.”
> >>>
> >>> It's easy to implement, and super useful. The TIA
> >>> has multiple ranges, each with its own output, but
> >>> multiple ranges are active at once; there's no loss
> >>> of data as would happen switching range resistors.
> >>>
> >>> Stephen and his co-authors found a simple, clever
> >>> trick to prevent input TIA opamp saturation, using
> >>> JFETs to successively short series-placed higher-
> >>> value range resistors for strong input currents.
> >>>
> >>> They suggest a three-stage implementation, with a
> >>> 300:1 ratio for each, but you can use many stages
> >>> (each one takes few extra parts), to obtain high
> >>> accuracy with a say 12-bit ADC. Also, a high
> >>> input-opamp Vos needn't degrade the dynamic range.
> >>>
> >>> DropBox has a draft of our x-Chapters write-up:
> >>> https://www.dropbox.com/s/fs4edz7dqgwswoj/4x.3.7_Eckel_TIA.pdf?dl=0
> >>>
> >>> I think you can download Stephen's RSI article here:
> >>> https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&ved=2ahUKEwjk6KvVwYfjAhVlRN8KHaMfD48QFjAHegQICRAC&url=http%3A%2F%2Fwww.bmo.physik.uni-muenchen.de%2F~riedle%2FElektronik_I%2FKW103%2F2012_Eckel%2CSushkov_9-decade_RSI.pdf&usg=AOvVaw3g9i6-pWAwuuZlZLvlFArd
> >>>
> >>>
> >>
> >>
> >> Marvellous - thank you!
> >>
> >> Jfets are nice because when fully depleted off there is no parasitic s-d
> >> diode that curses gumdrop mosfets. But gumdrop mosfets are much cheaper
> >> than jfets these days. I wonder if one could use series connected
> >> source-source 2N7002 and s-s BSS84 to replace those jfets? Eight mosfets
> >> could work out cheaper than four jfets? While gate leakage should be a
> >> non-issue I don't know how s-d channel leakage compares?
> >>
> >> piglet
> >
> > I was going to ask what about source-source fets, and found this,
> > https://electronics.stackexchange.com/questions/79028/understanding-two-mosfet-with-sources-connected
> >
> > Is that right... just looking at pics not reading comments/ words.
> >
> > George H.
> >
>
> Yes, here is sketch showing how might be done using mosfets to replace
> jfets:
>
> <https://www.dropbox.com/s/bidm47zj29moq6k/SteppedTIA.jpg?dl=0>
>
> piglet
Thanks piglet. I didn't know that Jfet's have no S-D body diode.
(Why the heck is that? so much I don't know!)
George H.
Reply by Winfield Hill●June 27, 20192019-06-27
piglet wrote...
>
>On 27/06/2019 2:10 pm, Winfield Hill wrote:
>> piglet wrote...
>>>
>>> On 27/06/2019 2:06 am, George Herold wrote:
>>>> On June 26, 2019, piglet wrote:
>>>>>
>>>>> Jfets are nice because when fully depleted off there is no parasitic s-d
>>>>> diode that curses gumdrop mosfets. But gumdrop mosfets are much cheaper
>>>>> than jfets these days. I wonder if one could use series connected
>>>>> source-source 2N7002 and s-s BSS84 to replace those jfets? Eight mosfets
>>>>> could work out cheaper than four jfets? While gate leakage should be a
>>>>> non-issue I don't know how s-d channel leakage compares?
>>>>
>>>> I was going to ask what about source-source fets, and found this,
>>>>https://electronics.stackexchange.com/questions/79028/understanding-two-mosfet-with-sources-connected
>>>>
>>>> Is that right... just looking at pics not reading comments/ words.
>>>
>>> Yes, here is sketch showing how might be done using
>>> mosfets to replace jfets:
>>>
>>> <https://www.dropbox.com/s/bidm47zj29moq6k/SteppedTIA.jpg?dl=0>
>>
>> I'm not sure you need the 2nd MOSFET. With MOSFET
>> sources to the opamp output, all MOSFETs are off
>> for voltages within your +/-(9V + Vgs-th) window.
>> A MOSFET problem is finding parts with low Coss.
>> E.g., BSS123 and BSS84 are about 7pF. This is
>> across your highest-value feedback resistor.
>
> But without the 2nd mosfet you'd effectively have
> anti-parallel diodes across the feedback resistors
> and be limited to 0.6V?
OK, good point, for a bipolar version. But OK for
unipolar. But I'd just add an ordinary signal diode.
> Good point about capacitance.
Carried to the extreme, it rules out JFETs too,
and one would have to use John's or Phil's scheme.
--
Thanks,
- Win
Reply by Jan Panteltje●June 27, 20192019-06-27
On a sunny day (Thu, 27 Jun 2019 06:52:16 -0700) it happened John Larkin
<jjlarkin@highlandtechnology.com> wrote in
<usg9hehn975p0k6sm43ous31hla2g17neq@4ax.com>:
>One day I want to try to visibly light an LED from our ambient RF.
I have done that, connect a LED to my CB GPA antenna,
it lit up when neighbor across the street used his set.
I called him, asked how many watts he was using.
eeehh
May also work with your cellphone if close enough.
Reply by John Larkin●June 27, 20192019-06-27
On 27 Jun 2019 05:19:44 -0700, Winfield Hill <winfieldhill@yahoo.com>
wrote:
>George Herold wrote...
>>
>> We sell a PD with a 10 position switch, 1,3.3,10...
>> It's OK, when I tried to make it faster I found
>> the switch added ~5 pF of C. Otherwise I'm not
>> sure what's wrong with a gain switch.
>
> Well, as I said, the Eckel scheme, with outputs
> running simultaneously, insures fast, accurate
> digitizing, especially with noisy signals. But
> John's scheme looks very practical for separating
> a high Rf first stage from a lower Rf second stage.
> The first stage would not suffer from any extra
> capacitances to slow it down. You could safely
> add switches to the lower-gain stage to get lots
> of ranges. I might use 10x for 1st to 2nd stage,
> to insure speed, and do the 3.3x bit later.
I am currently designing a fiberoptic clock distribution system, and
want to light an LED if there is activity on an ECL differential
signal pair. The cheap way to do that is with an AC-coupled diode
detector and a comparator, with very small pickoff caps driving the
diodes. That gets me into understanding the behavior of low-barrier
schottky diodes (SMS7621, BAT15) near zero volts. My little
multi-range photodiode amp also cares about "sub-threshold" diode
behavior.
I did low-voltage measurement on an SMS7621 yesterday, but it ocurrs
to me now that I may have been detecting ambient RF, which is after
all what those diodes are made for. I should do it again, without the
leads to power supplies and DVMs acting like nice antannas.
One day I want to try to visibly light an LED from our ambient RF.
I wonder how good the diode Spice models are at, say, +-0.5 volts,
especially close to zero volts. I guess I'll find out. The data sheets
are "RF", all about silly things like sine wave dBms.
The more expensive, and less interesting, way to do my signal detector
is digitally, namely to clock an EP52 flop off my diff pair and reset
it periodically or something.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics