DIY giga-ohm resistors???

[mike]

I built a crude electrometer.
Poke the gate of a jfet in the air and read the
drain current.
Uses a neon bulb as input protection and gate bias.

Problem is that it's WAY too sensitive for what I want.

100M resistor to ground kills the sensitivity.
Looks like I need something on the order of a few Gigahoms.

Anybody experienced with DIY BIG resistors?

About an inch of pencil line on card stock is about right.
But that's gonna be very unstable.

Best I've come up with so far is sheet magnetic material
used for stick-up business cards.

It's dimensionally stable enough to bolt on a contact.
Two parallel strips shunted by another strip
makes a variable resistor. I expect it's gonna
be unstable too, but at least it's adjustable.

Better ideas?

NO, I'm not gonna buy a resistor. I'll be done with
this thing before tomorrow's mail.

Thanks, mike

[John Larkin]

You don't kill an electrometer sensitivity with a resistor. Use a
capacitor.


--

John Larkin, President
Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation

[mike]

On 2/28/2012 6:56 PM, John Larkin wrote:
> On Tue, 28 Feb 2012 18:24:12 -0800, mike<spa...@gmail.com> wrote:
>
>> I built a crude electrometer.
>> Poke the gate of a jfet in the air and read the
>> drain current.
>> Uses a neon bulb as input protection and gate bias.
>>
>> Problem is that it's WAY too sensitive for what I want.
>>
>> 100M resistor to ground kills the sensitivity.
>> Looks like I need something on the order of a few Gigahoms.
>>
>> Anybody experienced with DIY BIG resistors?
>>
>> About an inch of pencil line on card stock is about right.
>> But that's gonna be very unstable.
>>
>> Best I've come up with so far is sheet magnetic material
>> used for stick-up business cards.
>>
>> It's dimensionally stable enough to bolt on a contact.
>> Two parallel strips shunted by another strip
>> makes a variable resistor. I expect it's gonna
>> be unstable too, but at least it's adjustable.
>>
>> Better ideas?
>>
>> NO, I'm not gonna buy a resistor. I'll be done with
>> this thing before tomorrow's mail.
>>
>> Thanks, mike
>
>
> You don't kill an electrometer sensitivity with a resistor. Use a
> capacitor.
>
>

Thanks, excellent advice. Certainly cuts the gain.
Works great for what it is.

Next experiment is to build a field mill.
Don't think I can stand the increased capacitance in a field mill.
And need it to come off the peg back into the dynamic range of the
amplifier...eventually. I think I need a resistor. But this is
all new territory for me.

Still want ideas on making BIG resistors.
mike

[Robert Baer]

90V strike voltage is NO "protection"...
Even the 65V operating voltage is NO "protection"...

[Jan Panteltje]

On a sunny day (Tue, 28 Feb 2012 18:24:12 -0800) it happened mike
<spa...@gmail.com> wrote in <jik28t$r40$1...@dont-email.me>:

I did put 10 110 MOhm SMDs in series to get 1.1 GOhm, here:
http://panteltje.com/pub/PMT_regulated_power_supply_diagram_img_3182.jpg

On the left side:
http://panteltje.com/pub/PMT_HV_supply_componet_side_img_3180.jpg

Works fine, you are limited by the maximum voltage those
things are specified for.

[mike]

OK, what would you suggest?

Strike voltage is about 70V in the preferred direction.
That limits the gate current to 4 microamps reverse.
Suppose that's excessive?

I bit the bullet and put back-back zeners at the gate.
I can see the added leakage, but
I wanted some leakage to ground anyway.

[Spehro Pefhany]

Digikey stocks 10G 5% resistors for less than $4. HVF1206T1008JE, and
others.

Or, if you really feel the need to make something, maybe your pencil
line on something like this:

http://www.partsconnexion.com/terminal_connex.html

(ceramic base).


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
sp...@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

[Phil Hobbs]

When I was but a stripling in my first engineering job (before grad
school) one of my first tasks was to figure out the right bias resistor
for a coax-in, waveguide-out, SRD doubler using small ceramic devices
with gold plated end caps. (IIRC they had been using a Microwave
Associates one and wanted to qualify a second source, Gigahertz
Devices.)

The doublers were self-biased, i.e. rectification provided the right
bias to get the best snap, so the bias network was just a parallel RC
across the diode. The diode mount and waveguide provided the
capacitance, and the resistor in the production units was a small glob
of thick-film resistor goo right on the side of the ceramic diode case.

To find out what that resistor should be, the trick was to scribble on
the side of a clean diode with an HB pencil, measure the resistance,
plot the 2f vs 1f power conversion efficiency, scribble some more,
repeat. It worked pretty well, except that I had to reduce the input
power range to avoid burning the graphite during data taking. (The new
resistor was something like 200k vs 120k for the old one.)

So there are actual uses for pencil-on-ceramic resistors.

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

[Bernhard Kuemel]

On 02/29/2012 03:24 AM, mike wrote:
> Anybody experienced with DIY BIG resistors?

Not much.

Maybe distilled water.

Or a gas gap exposed to ionizing radiation. Water vapor and a UV LED
might work, but I don't know the required wavelength. Should be
adjustable by the amount of ionizing radiation or the electrode
distance. Pressure should be high enough to avoid avalanche break
through. You could try to use a smoke detector that uses americium as
source of ionizing radiation. Uranium is fairly easy to acquire, but has
a low activity. There are professional radiation sources available, too.
Betalights come to my mind. They contain radioactive tritium. You would
have to break open the vial, though, to make use of the tritium, as the
radiation won't penetrate the vial.

A 1N4148 diode has 0.8 GOhm reverse resistance at 20 V and 25 degC. You
can certainly find diodes with higher resistances. The resistance
depends on voltage and temperature, though, and is DC only.

Bernhard

[Sjouke Burry]

mike <spa...@gmail.com> wrote in news:jikf1r$f9a$1...@dont-email.me:

Take a 4-8 inch high transmitter tube, power up the
cathode keating, and use a negative voltage on the grid
to tune for the resistance you want.
In a pinch an audio endstage tube will do.

[Robert Baer]

If the gate oxide breaks down at 15V (typical) youall aint gott no
perrtektion.
STM makes a number of FETs with integrated back-to-back zeners for
protection.
Added leakage can be in the nanoamp or less region at 25C.

[Robert Baer]

OOooh! Shades of the old tube SexTronix 'scopes!

[Robert Baer]

Suggest you experiment with the plate voltage at a given grid voltage..

[Bill Beaty]

On Feb 28, 6:24 pm, mike <spam...@gmail.com> wrote:

> Better ideas?

I once did this using india ink on painted acrylic. The ink is a
carbon resistor, and you can dilute it to produce large values.

One problem was with the junction from ink to electrodes. Better use
conductive silver ink as terminals, then run the india ink up over the
silver.

Here's one I haven't tried: neon glow lamps with low turn-on voltage,
where thorium in the electrodes provides ion leakage.

http://www.techlib.com/science/ion.html#Neon%20as%20Resistor
http://tubetime.us/?p=27

As they note, you'll need to clean the glass, heat it to de-sorb any
water, then cover it with a coat of insulating paint.


Another possibility, from Phil Hobbs' $10 thermal imager paper: two
LEDs facing each other, covered in something opaque. One acts as a
(dim) light source, the other is a photodiode. If you pump some uA
into the emitter, will the PD give some pA of bias current?

[Jan Panteltje]

On a sunny day (01 Mar 2012 01:48:10 GMT) it happened Sjouke Burry <s@b> wrote
in <XnsA0091C7DA8C3Bsj...@213.75.12.10>:

>Take a 4-8 inch high transmitter tube, power up the
>cathode keating, and use a negative voltage on the grid
>to tune for the resistance you want.
>In a pinch an audio endstage tube will do.

This only works for triodes (resistor like Va/Ia curve).
Penthodes have a constant current behavior.
Most audio output stage tubes are penthodes.

[dagmarg...@yahoo.com]

Back-to-back LEDs are much better. Keep 'em dark.

--
Cheers,
James Arthur

[Spehro Pefhany]

That's normal for an abs. max rating, but I wonder what the typical
actual breakdown voltage is. For example, a Calogic 2N7002 has an abs
max rating of +/-40V, I would guess that the typical breakdown is more
like 100VDC, so a neon might be okay.

> STM makes a number of FETs with integrated back-to-back zeners for
>protection.
> Added leakage can be in the nanoamp or less region at 25C.

A nA can be huge. According to measurements made by skilled
practitioners (eg. Win Hill) actual leakage of a 2N700x is a few fA.

[mike]

I started by asking about making big resistors.
This has morphed into a party where every drunken cowboy is shooting his
gun in all directions.

Let's put a target on the wall and encourage everybody to AIM.

The question has become, "what does it take to damage the gate of a JFET.
(2n700x is a MOSFET)

Neon lamp is across the input.
10Meg series resistor to gate of 2N4416 JFET. Why a 2N4416?
Simple, that's what I could find in the junk box, and it works.
7K in the source, Meter in the Drain, 9V battery.

Neon fires at about 70V (in the dark) if you stick it in the right
direction. About 90V in the other direction
Gate breakdown voltage spec is 30V.

In the forward direction, I've got about 9 microamps of gate current.
That shouldn't be harmful???

In the reverse direction, I've got about 4 microamps of gate current
at -30V.
So, the basic question is, just that; "is 4 microamps at -30V on the
gate harmful in this application?"
I'm not worried about increasing the noise figure.
I am worried about significant increases in gate leakage current at ~0V.

For now, let's just ignore all the nasty parasitic and transient things
that happen if you actually strike an arc at the input. That's a whole
other bucket of worms.

Ready...AIM...

[be...@iwaynet.net]

On 3/1/2012 4:33 PM, mike wrote:

> I started by asking about making big resistors.
> This has morphed into a party where every drunken cowboy is shooting his
> gun in all directions.
>
> Let's put a target on the wall and encourage everybody to AIM.

Yee Haw! Hey if you want a STABLE high value resistor it ain't so
simple. First you have to be very careful of the insulator. Glass (which
includes ceramic) is OUT. The absolute best is polystyrene. For quick
and dirty a pencil line on mylar drawing paper isn't too bad.

In a case where I needed very high resistance with reasonable accuracy,
we built a circuit board with hundreds of high value surface mount
resistors in series. Handles voltage, low leakage (with teflon circuit
board) and accurate. We only had one blow itself to kingdom come. Don't
know why it failed.

> The question has become, "what does it take to damage the gate of a JFET.
> (2n700x is a MOSFET)
>
> Neon lamp is across the input.
> 10Meg series resistor to gate of 2N4416 JFET. Why a 2N4416?
> Simple, that's what I could find in the junk box, and it works.
> 7K in the source, Meter in the Drain, 9V battery.
>
> Neon fires at about 70V (in the dark) if you stick it in the right
> direction. About 90V in the other direction
> Gate breakdown voltage spec is 30V.

> For now, let's just ignore all the nasty parasitic and transient things
> that happen if you actually strike an arc at the input. That's a whole
> other bucket of worms.
>
> Ready...AIM...

Yes, I've tried the neon "protection" thing. Basically worthless.
Problem as noted above is transient response. And anyway, don't you know
that the gates of FETs of ANY type make better fuses than ANY protection
circuit?

[Phil Hobbs]

That's how SED works--we all discuss what we feel like discussing, just
the same as you. If you want people to do something specific for you,
pay them or else be helpful and friendly yourself so that they want to
do it for free.

[Robert Baer]

Do you have to worry about dark current like in PMTs?
Do you feed them with s* or otherwise treat them like mushrooms?
Does it help to use black light LEDs?

[dagmarg...@yahoo.com]

On Friday, March 2, 2012 12:25:30 AM UTC-5, Robert Baer wrote:

Not much, that's the appeal. You've got to worry about their photocurrent, hence the need to keep 'em dark.

Good LEDs have amazingly low forward current at low forward bias. IIRC there's a nice graph out there in one of Pease's articles.

Mike could use a couple to clamp Vgs to the rails, or 2x2 in series, back-to-back.

> Do you feed them with s* or otherwise treat them like mushrooms?
> Does it help to use black light LEDs?

I think UV LEDs are leakier. I'm not sure which LED materials / colors are best--haven't measured recently.

Another junk-box low-leakage item he could use to protect the gate: bipolar transistor junctions. Much better than, e.g., 1n4148.

--
Cheers,
James Arthur

[George Herold]

> James Arthur- Hide quoted text -
>
> - Show quoted text -

Hi James, Knowing almost nothing about the reverse leakage current of
leds, I'd guess that the higher the energy of the photon the lower the
leakage... a kT vs. E-gap thing.

Leaky uV diodes don't fit this model.... which might be curious.

George H.

[George Herold]

> George H.- Hide quoted text -

>
> - Show quoted text -

Oops, and the low forward voltage current (re-reading...)

[dagmarg...@yahoo.com]

I figured the same, but Phil said something a while back about excess lattice defects in certain LEDs (blue GaN?) that gave me pause.

I used green GaN as low-leakage clamps, mostly for the high Vf. They were very good, but I didn't check leakage down to electrometer current levels.

--
Cheers,
James Arthur

[Phil Hobbs]

Normal red and green LEDs are very good. I haven't measured any just
lately, but a dozen or so years ago when I was doing it for real, I
measured several jellybean red AlGaAs display LEDs at less than 50 fA
from -5V to +0.5V. I think they were from Chicago Miniature Lamp--I got
a couple of reels, and haven't run out yet.

[George Herold]

Yeah I think that's right... I heard of this physics lab where you
look at the 'turn on' voltage of LEDs, vs the wavelength... and things
work out 'as expected' till you get to the blues...

Where it doesn't... I've not done the measurements.

>
> I used green GaN as low-leakage clamps, mostly for the high Vf.  They were very good, but I didn't check leakage down to electrometer current levels.

Hmm, that's interesting. Are the blues 'worse' than the greens? Or
do they just not fit 'expected value' as well?

George H.

[dagmarg...@yahoo.com]

On Friday, March 2, 2012 6:33:26 PM UTC-5, George Herold wrote:
> On Mar 2, 6:11 pm, dagmargoodb...@yahoo.com wrote:
> > On Friday, March 2, 2012 5:59:44 PM UTC-5, George Herold wrote:
> > > On Mar 2, 5:57 pm, George Herold <gher...@teachspin.com> wrote:
> > > > On Mar 2, 5:37 pm, dagmargoodb...@yahoo.com wrote:
> >
> > > > > On Friday, March 2, 2012 12:25:30 AM UTC-5, Robert Baer wrote:
> > > > > > dagmargoodboat wrote:
> > > > > > > On Wednesday, February 29, 2012 6:25:04 AM UTC-5, mike wrote:

> > > > > > >> I bit the bullet and put back-back zeners at the gate.
> > > > > > >> I can see the added leakage, but
> > > > > > >> I wanted some leakage to ground anyway.
> >
> > > > > > > Back-to-back LEDs are much better.  Keep 'em dark.
> >
> > > > > >    Do you have to worry about dark current like in PMTs?
> >
> > > > > Not much, that's the appeal.  You've got to worry about their photocurrent, hence the need to keep 'em dark.
> >
> > > > > Good LEDs have amazingly low forward current at low forward bias.  IIRC there's a nice graph out there in one of Pease's articles.
> >
> > > > > Mike could use a couple to clamp Vgs to the rails, or 2x2 in series, back-to-back.
> >
> > > > > >    Do you feed them with s* or otherwise treat them like mushrooms?
> > > > > >    Does it help to use black light LEDs?
> >
> > > > > I think UV LEDs are leakier.  I'm not sure which LED materials / colors are best--haven't measured recently.
> >
> > > > > Another junk-box low-leakage item he could use to protect the gate: bipolar transistor junctions.  Much better than, e.g., 1n4148.
> >
> >

> > > > Hi James,  Knowing almost nothing about the reverse leakage current of
> > > > leds, I'd guess that the higher the energy of the photon the lower the
> > > > leakage... a kT vs. E-gap thing.
> >
> > > > Leaky uV diodes don't fit this model.... which might be curious.
> >
> > > > George H.
> >
> >

> > > Oops, and the low forward voltage current (re-reading...)
> >
> > I figured the same, but Phil said something a while back about excess lattice defects in certain LEDs (blue GaN?) that gave me pause.
>
> Yeah I think that's right... I heard of this physics lab where you
> look at the 'turn on' voltage of LEDs, vs the wavelength... and things
> work out 'as expected' till you get to the blues...
>
> Where it doesn't... I've not done the measurements.
>
> >
> > I used green GaN as low-leakage clamps, mostly for the high Vf.  They were very good, but I didn't check leakage down to electrometer current levels.
>
> Hmm, that's interesting. Are the blues 'worse' than the greens? Or
> do they just not fit 'expected value' as well?

I measured a number of eBay whites (=blue GaN + phosphor) that were very leaky in the forward direction (manifests as a high threshold). They might've been static damaged, or who knows what, but it was consistent. I assume they would've leaked backwards too. Dunno if that's common, but there are behaviors in prime parts (low-current flickering) that hint it could be.

So, caveat emptor.

--
Cheers,
James Arthur

[dagmarg...@yahoo.com]

On Friday, March 2, 2012 6:42:40 PM UTC-5, Phil Hobbs wrote:

> dagmargoodboat wrote:
> >
> > On Friday, March 2, 2012 5:59:44 PM UTC-5, George Herold wrote:
> > > On Mar 2, 5:57 pm, George Herold <gher...@teachspin.com> wrote:
> > > > On Mar 2, 5:37 pm, dagmargoodb...@yahoo.com wrote:
> > > > > On Friday, March 2, 2012 12:25:30 AM UTC-5, Robert Baer wrote:
> > > > > > dagmargoodboat wrote:
> > > > > > > On Wednesday, February 29, 2012 6:25:04 AM UTC-5, mike wrote:

> > > > > > >> I bit the bullet and put back-back zeners at the gate.
> > > > > > >> I can see the added leakage, but
> > > > > > >> I wanted some leakage to ground anyway.
> > > >
> > > > > > > Back-to-back LEDs are much better. Keep 'em dark.
> > > >
> > > > > > Do you have to worry about dark current like in PMTs?
> > > >
> > > > > Not much, that's the appeal. You've got to worry about their photocurrent, hence the need to keep 'em dark.
> > > >
> > > > > Good LEDs have amazingly low forward current at low forward bias. IIRC there's a nice graph out there in one of Pease's articles.
> > > >
> > > > > Mike could use a couple to clamp Vgs to the rails, or 2x2 in series, back-to-back.
> > > >
> > > > > > Do you feed them with s* or otherwise treat them like mushrooms?
> > > > > > Does it help to use black light LEDs?
> > > >
> > > > > I think UV LEDs are leakier. I'm not sure which LED materials / colors are best--haven't measured recently.
> > > >
> > > > > Another junk-box low-leakage item he could use to protect the gate: bipolar transistor junctions. Much better than, e.g., 1n4148.
> > > >
> > > >
> > > >

> > > > Hi James, Knowing almost nothing about the reverse leakage current of
> > > > leds, I'd guess that the higher the energy of the photon the lower the
> > > > leakage... a kT vs. E-gap thing.
> > > >
> > > > Leaky uV diodes don't fit this model.... which might be curious.
> > > >
> > > > George H.
> > > >
> > >

> > > Oops, and the low forward voltage current (re-reading...)
> >
> > I figured the same, but Phil said something a while back about excess lattice defects in certain LEDs (blue GaN?) that gave me pause.
> >
> > I used green GaN as low-leakage clamps, mostly for the high Vf. They were very good, but I didn't check leakage down to electrometer current levels.
>

> Normal red and green LEDs are very good. I haven't measured any just
> lately, but a dozen or so years ago when I was doing it for real, I
> measured several jellybean red AlGaAs display LEDs at less than 50 fA
> from -5V to +0.5V. I think they were from Chicago Miniature Lamp--I got
> a couple of reels, and haven't run out yet.

Here's some old data from Win...


<quote>
------------------------
In article <3B882B44...@rowland.org>,
Winfield Hill wrote:

> Here are a set of I-V measurements for your edification.
>
> -- Schottky -- -- silicon -- --- JFET ---
> 1n5711 1n4148 red
> 1n5819 1n6263 1n4153 1n458 PAD-1 2n4117 LED
> ----- ------ ------ ----- ----- ----- ----
> 10mA 270 700 750 800 1330 940 1706
> 1 200 375 620 670 810 790 1614
> 100uA 130 280 510 577 710 710 1535
> 10 60 210 375 490 615 640 1425
> 1 23 80 255 415 555 580 1330
> 100nA 0 10 140 330 500 525 1205
> 10 - 0 65 255 440 460 1080
> 1 - - 12 185* 370 393 950
> 100pA - - 1.3* 125* 300 335 840*
> 10 - - * 70* 230 270 *
> 1 - - ** * 155 220 *

* indicates affected by light pickup. I put some curves on the web,
http://people.ne.mediaone.net/whill/elec/comp/diode/diode-curves.html

Note how ordinary glass silicon diodes look like 10M resistors
below +/-50mV. GROSS! but often acceptable for virtual-ground node
protection, e.g. if the feedback resistor is 1M or less, etc.

> Interesting to compare these numbers with typical LMC6001 op amp
> inputs. I believe Bob Pease says he and his techs have never seen
> more than about 5fA input bias current (presumably at room temp) on
> any of the hundreds they've looked at. The data sheet suggests that
> the inputs conduct when taken beyond the power rails, and can stand a
> few mA of current. I wonder if they could be used as a super low
> leakage diode! (In very special circumstances, of course.)
------------------------
</quote>

--
Cheers,
James Arthur

[Robert Baer]

Be advised the last two lines were stated in jest..

[Phil Hobbs]

Not at all. BFT25A B-C junctions are competitive with the best
low-leakage silicon diodes anywhere.

[Robert Baer]

Phil Hobbs wrote:
>> Do you feed them with s* or otherwise treat them like mushrooms?
>> > >> Does it help to use black light LEDs?
>> >

Err..i was referring to my comments stated above.

[dagmarg...@yahoo.com]

Yeah, I knew, but there are "black light LEDs," so it seemed worth
commenting on.

--
Cheers,
James Arthur

[k...@att.bizzzzzzzzzzzz]

Dark Emitting Axial Diodes?

[John Larkin]

Hey, Phil, do you know anything about the reverse beta of a BFT25?

Imagine a linear ramp made from a current source and a cap; say 10 mA
and 20 pF, for a few ns to 2.5 volts. Now I want to switch in bigger
caps for slower ranges, like 200 pF and 2 nF. I figured I could use
BFT25s to do that. But when I discharge the ramp, current flows
backwards into the BFT25s.

isrc
|
v
|
| diode
+----------+---------+---ak----reset
| | |
| | |
20pF 200p 2n
| | |
| | |
| | |
gnd c c
>--b >--b
e e
| |
gnd gnd

If there's any decent amount of reverse beta, I don't need a huge
amount of base current to get a decent reset time.

(I'll probably add a weak pullup to the transistor collectors to bias
them up when they're off.)

I have some very rough data that suggests a reverse beta around 5
maybe, but it's not solid.


--

John Larkin, President Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

[Ken S. Tucker]

Bill Beaty wrote:
> On Feb 28, 6:24 pm, mike <spam...@gmail.com> wrote:
>
>> Better ideas?
>
> I once did this using india ink on painted acrylic. The ink is a
> carbon resistor, and you can dilute it to produce large values.
>
> One problem was with the junction from ink to electrodes. Better use
> conductive silver ink as terminals, then run the india ink up over the
> silver.

Good idea that India ink. Stir some into some plastercine to put
in a pair of wires for a variable resistor.
Stir into glue, wires at each end, let dry and measure.
Ken

[Jamie]

Clamp diode of low pF from C to E on each one? You could remove the
20pF and use what is in the diodes for that?


Jamie

[Jan Panteltje]

On a sunny day (Sun, 04 Mar 2012 09:54:50 -0500) it happened Jamie
<jamie_ka1lpa_not_v...@charter.net> wrote in
<thL4r.36816$rS.2...@newsfe15.iad>:

Is this not normaly done with PIN diodes?

[Phil Hobbs]

According to the spice model, it's about 16.

[John Larkin]

Yes, that would absorb most of the discharge current. Tiny schottkies
wouldn't add much capacitance. We have some nice diodes in stock,
SMS7621, that are only about 0.25 pF.

You could remove the
>20pF and use what is in the diodes for that?

The diode capacitances are nonlinear on voltage, so that would bend my
nice linear ramp. That's the reason to pull up the "off" transistor
collectors, to reduce the nonlinearity of their c-e capacitance.

[Jamie]

Yes, you are so correct there.:)

Jamie

[dagmarg...@yahoo.com]

A relative of N.E.D.s, high-power noise emitting diodes, which emit a loud noise, once. This transforms them into DEADs.

James