Anyone make PCBs with Othermill?

[Phil Hobbs]

On 7/10/2015 12:11 AM, Tim Williams wrote:

"Clifford Heath" <no.spam@please.net> wrote in message
news:GpHnx.142982$_a4.19916@fx03.iad...
On 10/07/15 13:10, Phil Hobbs wrote:
It's simpler than that--drain impedance is just the reciprocal of the
drain admittance, i.e. 1/(dI_d/dV_ds).

In BJTs, collector impedance is Early voltage divided by collector
current. Normally that's a big number.

Oh, it seems I misinterpreted, and talked about the output impedance.

Is it valid interpretation to say that the "drain impedance" is the
impedance that appears in parallel with the drain resistor (assuming
that's what it is) to form the output impedance?

Yes.

See also: plate resistance.

Phil, do you have any hints at "plate curves" for these beasts? Have
triodes actually returned, in the sense of being a (at least somewhat)
constant voltage plate/drain characteristic?

Haven't measured, but they sure make crappy followers.

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

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

 

[Phil Hobbs]

On 07/10/2015 04:31 AM, Tom Swift wrote:

Phil Hobbs <hobbs@electrooptical.net> wrote:

SiGe:C transistors such as the BFP640 have effectively infinite
collector resistance, so cascoding pHEMTs with SiGe:C BJTs gives you
the best of all worlds: noise temperatures below 30 K and high, linear
gain.

Cheers

Phil Hobbs

Phil, here is a related question.

In the thread "Re: emitter follower gain", you posted the following:

A 40-GHz transistor with super low capacitance, high beta, and a
practically infinite V_A lets you do magical things, eg a cascoded
pHEMT front end with 0.3 nV 1-Hz noise, < 1 pF of capacitance, and
an RC-coupled voltage gain of 50 from DC to some ridiculous
frequency.

(The pHEMT has a 1/f corner of ~10 MHz, unfortunately.)

I posted a circuit like that a year or so back, in the
"electrometer front end" thread.

The post is http://tinyurl.com/pdw6nwu

I searched through the thread you referenced and could not find the
circuit. I searched google groups for all the LTspice circuits you
posted, and searched your web site, but could not find it.

Do you have a copy of the circuit and if so, could you post it again?

Thanks

The subthread about the circuit begins with a post whose time stamp is
06/05/2014 01:09 PM. On Google it's
https://groups.google.com/d/msg/sci.electronics.design/5nqKFRv_HnI/Ig9tzZBKx-gJ

or

http://tinyurl.com/okwupty .

The actual circuit is slightly different, because the SPICE model abuses
the CFA output stage horribly, but this is the gist. It lives and dies
by keeping the input capacitance very very low, like wire-bonding the
biochip right to the board.

Worked great, and almost made the ridiculous spec.

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

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

 

[John Larkin]

On Fri, 10 Jul 2015 08:40:51 -0400, Phil Hobbs
<hobbs@electrooptical.net> wrote:

On 7/10/2015 12:11 AM, Tim Williams wrote:
"Clifford Heath" <no.spam@please.net> wrote in message
news:GpHnx.142982$_a4.19916@fx03.iad...
On 10/07/15 13:10, Phil Hobbs wrote:
It's simpler than that--drain impedance is just the reciprocal of the
drain admittance, i.e. 1/(dI_d/dV_ds).

In BJTs, collector impedance is Early voltage divided by collector
current. Normally that's a big number.

Oh, it seems I misinterpreted, and talked about the output impedance.

Is it valid interpretation to say that the "drain impedance" is the
impedance that appears in parallel with the drain resistor (assuming
that's what it is) to form the output impedance?

Yes.

See also: plate resistance.

Phil, do you have any hints at "plate curves" for these beasts? Have
triodes actually returned, in the sense of being a (at least somewhat)
constant voltage plate/drain characteristic?

Haven't measured, but they sure make crappy followers.

A 60 GHz source follower would be interesting.

If you're lucky enough to get actual DC curves (and not just
s-params), some phemts have downward-sloping drain curves, namely
negative output impedance. Maybe the effect is thermal.



--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

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

 

[Phil Hobbs]

On 07/10/2015 10:42 AM, John Larkin wrote:

On Fri, 10 Jul 2015 08:40:51 -0400, Phil Hobbs
hobbs@electrooptical.net> wrote:

On 7/10/2015 12:11 AM, Tim Williams wrote:
"Clifford Heath" <no.spam@please.net> wrote in message
news:GpHnx.142982$_a4.19916@fx03.iad...
On 10/07/15 13:10, Phil Hobbs wrote:
It's simpler than that--drain impedance is just the reciprocal of the
drain admittance, i.e. 1/(dI_d/dV_ds).

In BJTs, collector impedance is Early voltage divided by collector
current. Normally that's a big number.

Oh, it seems I misinterpreted, and talked about the output impedance.

Is it valid interpretation to say that the "drain impedance" is the
impedance that appears in parallel with the drain resistor (assuming
that's what it is) to form the output impedance?

Yes.

See also: plate resistance.

Phil, do you have any hints at "plate curves" for these beasts? Have
triodes actually returned, in the sense of being a (at least somewhat)
constant voltage plate/drain characteristic?

Haven't measured, but they sure make crappy followers.

A 60 GHz source follower would be interesting.

I built a freebie calibrator circuit into that amplifier, because (a) it
needed one, and (b) I wanted to try out those parts in various roles.

I eventually got the circuit working by replacing the ATF38143 follower
with a SKY65050. The Skyworks part is generally nicer in circuitry,
except that its flatband noise is a decibel or two higher, and its 1/f
corner is 50 MHz rather than 10 MHz, at least in my small sample.

If you're lucky enough to get actual DC curves (and not just
s-params), some phemts have downward-sloping drain curves, namely
negative output impedance. Maybe the effect is thermal.

I've seen that in SiGe:C datasheets, but not in pHEMTs. Do you remember
which ones? That effect could be pretty useful if it's fast.

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

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

 

[Phil Hobbs]

On 07/10/2015 10:49 AM, Phil Hobbs wrote:

On 07/10/2015 10:42 AM, John Larkin wrote:
On Fri, 10 Jul 2015 08:40:51 -0400, Phil Hobbs
hobbs@electrooptical.net> wrote:

On 7/10/2015 12:11 AM, Tim Williams wrote:
"Clifford Heath" <no.spam@please.net> wrote in message
news:GpHnx.142982$_a4.19916@fx03.iad...
On 10/07/15 13:10, Phil Hobbs wrote:
It's simpler than that--drain impedance is just the reciprocal of the
drain admittance, i.e. 1/(dI_d/dV_ds).

In BJTs, collector impedance is Early voltage divided by collector
current. Normally that's a big number.

Oh, it seems I misinterpreted, and talked about the output impedance.

Is it valid interpretation to say that the "drain impedance" is the
impedance that appears in parallel with the drain resistor (assuming
that's what it is) to form the output impedance?

Yes.

See also: plate resistance.

Phil, do you have any hints at "plate curves" for these beasts? Have
triodes actually returned, in the sense of being a (at least somewhat)
constant voltage plate/drain characteristic?

Haven't measured, but they sure make crappy followers.

A 60 GHz source follower would be interesting.

I built a freebie calibrator circuit into that amplifier, because (a) it
needed one, and (b) I wanted to try out those parts in various roles.

The circuit is posted somewhere in the "really triangular triangle wave"
thread.

I eventually got the circuit working by replacing the ATF38143 follower
with a SKY65050. The Skyworks part is generally nicer in circuitry,
except that its flatband noise is a decibel or two higher, and its 1/f
corner is 50 MHz rather than 10 MHz, at least in my small sample.

If you're lucky enough to get actual DC curves (and not just
s-params), some phemts have downward-sloping drain curves, namely
negative output impedance. Maybe the effect is thermal.



I've seen that in SiGe:C datasheets, but not in pHEMTs. Do you remember
which ones? That effect could be pretty useful if it's fast.

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

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

 

[John Larkin]

On Fri, 10 Jul 2015 10:49:58 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 07/10/2015 10:42 AM, John Larkin wrote:
On Fri, 10 Jul 2015 08:40:51 -0400, Phil Hobbs
hobbs@electrooptical.net> wrote:

On 7/10/2015 12:11 AM, Tim Williams wrote:
"Clifford Heath" <no.spam@please.net> wrote in message
news:GpHnx.142982$_a4.19916@fx03.iad...
On 10/07/15 13:10, Phil Hobbs wrote:
It's simpler than that--drain impedance is just the reciprocal of the
drain admittance, i.e. 1/(dI_d/dV_ds).

In BJTs, collector impedance is Early voltage divided by collector
current. Normally that's a big number.

Oh, it seems I misinterpreted, and talked about the output impedance.

Is it valid interpretation to say that the "drain impedance" is the
impedance that appears in parallel with the drain resistor (assuming
that's what it is) to form the output impedance?

Yes.

See also: plate resistance.

Phil, do you have any hints at "plate curves" for these beasts? Have
triodes actually returned, in the sense of being a (at least somewhat)
constant voltage plate/drain characteristic?

Haven't measured, but they sure make crappy followers.

A 60 GHz source follower would be interesting.

I built a freebie calibrator circuit into that amplifier, because (a) it
needed one, and (b) I wanted to try out those parts in various roles.

I eventually got the circuit working by replacing the ATF38143 follower
with a SKY65050. The Skyworks part is generally nicer in circuitry,
except that its flatband noise is a decibel or two higher, and its 1/f
corner is 50 MHz rather than 10 MHz, at least in my small sample.

If you're lucky enough to get actual DC curves (and not just
s-params), some phemts have downward-sloping drain curves, namely
negative output impedance. Maybe the effect is thermal.



I've seen that in SiGe:C datasheets, but not in pHEMTs. Do you remember
which ones? That effect could be pretty useful if it's fast.

Cheers

Phil Hobbs

Here it is in the CLY2, which is a mesfet:

https://dl.dropboxusercontent.com/u/53724080/Parts/Fets/CLY2_TQ.pdf

The negative slope corresponds to the high power dissipation region.
Maybe these are true non-pulsed DC measurements, so the effect could
be thermal. Just like some RF guys to do that.

I'm pretty sure I've seen the same in a phemt, but I can't find it
just now.

The DC specs are awful (in the sense of missing) on most RF parts. You
are lucky to get anything beyond Idss and drain breakdown voltage, and
you shouldn't believe the latter. The app notes literally say "bias it
until it works." We run supposedly 6 volt parts at 10, because they
actually start to leak at 25. That's RFthink again.


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

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

 

[Phil Hobbs]

On 7/10/2015 12:12 PM, John Larkin wrote:

On Fri, 10 Jul 2015 10:49:58 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 07/10/2015 10:42 AM, John Larkin wrote:
On Fri, 10 Jul 2015 08:40:51 -0400, Phil Hobbs
hobbs@electrooptical.net> wrote:

On 7/10/2015 12:11 AM, Tim Williams wrote:
"Clifford Heath" <no.spam@please.net> wrote in message
news:GpHnx.142982$_a4.19916@fx03.iad...
On 10/07/15 13:10, Phil Hobbs wrote:
It's simpler than that--drain impedance is just the reciprocal of the
drain admittance, i.e. 1/(dI_d/dV_ds).

In BJTs, collector impedance is Early voltage divided by collector
current. Normally that's a big number.

Oh, it seems I misinterpreted, and talked about the output impedance.

Is it valid interpretation to say that the "drain impedance" is the
impedance that appears in parallel with the drain resistor (assuming
that's what it is) to form the output impedance?

Yes.

See also: plate resistance.

Phil, do you have any hints at "plate curves" for these beasts? Have
triodes actually returned, in the sense of being a (at least somewhat)
constant voltage plate/drain characteristic?

Haven't measured, but they sure make crappy followers.

A 60 GHz source follower would be interesting.

I built a freebie calibrator circuit into that amplifier, because (a) it
needed one, and (b) I wanted to try out those parts in various roles.

I eventually got the circuit working by replacing the ATF38143 follower
with a SKY65050. The Skyworks part is generally nicer in circuitry,
except that its flatband noise is a decibel or two higher, and its 1/f
corner is 50 MHz rather than 10 MHz, at least in my small sample.

If you're lucky enough to get actual DC curves (and not just
s-params), some phemts have downward-sloping drain curves, namely
negative output impedance. Maybe the effect is thermal.



I've seen that in SiGe:C datasheets, but not in pHEMTs. Do you remember
which ones? That effect could be pretty useful if it's fast.

Cheers

Phil Hobbs

Here it is in the CLY2, which is a mesfet:

https://dl.dropboxusercontent.com/u/53724080/Parts/Fets/CLY2_TQ.pdf

The negative slope corresponds to the high power dissipation region.
Maybe these are true non-pulsed DC measurements, so the effect could
be thermal. Just like some RF guys to do that.

I'm pretty sure I've seen the same in a phemt, but I can't find it
just now.

The DC specs are awful (in the sense of missing) on most RF parts. You
are lucky to get anything beyond Idss and drain breakdown voltage, and
you shouldn't believe the latter. The app notes literally say "bias it
until it works." We run supposedly 6 volt parts at 10, because they
actually start to leak at 25. That's RFthink again.

Hmm, interesting. It does look like a thermal effect, since the slope
changes sign as you go to higher dissipation. The top curve drops about
100 mA in 6 volts, which would be a drain impedance of -60 ohms. That
would be interesting to stabilize!

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

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

 

[John Larkin]

On Fri, 10 Jul 2015 17:04:56 -0400, Phil Hobbs
<hobbs@electrooptical.net> wrote:

On 7/10/2015 12:12 PM, John Larkin wrote:
On Fri, 10 Jul 2015 10:49:58 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 07/10/2015 10:42 AM, John Larkin wrote:
On Fri, 10 Jul 2015 08:40:51 -0400, Phil Hobbs
hobbs@electrooptical.net> wrote:

On 7/10/2015 12:11 AM, Tim Williams wrote:
"Clifford Heath" <no.spam@please.net> wrote in message
news:GpHnx.142982$_a4.19916@fx03.iad...
On 10/07/15 13:10, Phil Hobbs wrote:
It's simpler than that--drain impedance is just the reciprocal of the
drain admittance, i.e. 1/(dI_d/dV_ds).

In BJTs, collector impedance is Early voltage divided by collector
current. Normally that's a big number.

Oh, it seems I misinterpreted, and talked about the output impedance.

Is it valid interpretation to say that the "drain impedance" is the
impedance that appears in parallel with the drain resistor (assuming
that's what it is) to form the output impedance?

Yes.

See also: plate resistance.

Phil, do you have any hints at "plate curves" for these beasts? Have
triodes actually returned, in the sense of being a (at least somewhat)
constant voltage plate/drain characteristic?

Haven't measured, but they sure make crappy followers.

A 60 GHz source follower would be interesting.

I built a freebie calibrator circuit into that amplifier, because (a) it
needed one, and (b) I wanted to try out those parts in various roles.

I eventually got the circuit working by replacing the ATF38143 follower
with a SKY65050. The Skyworks part is generally nicer in circuitry,
except that its flatband noise is a decibel or two higher, and its 1/f
corner is 50 MHz rather than 10 MHz, at least in my small sample.

If you're lucky enough to get actual DC curves (and not just
s-params), some phemts have downward-sloping drain curves, namely
negative output impedance. Maybe the effect is thermal.



I've seen that in SiGe:C datasheets, but not in pHEMTs. Do you remember
which ones? That effect could be pretty useful if it's fast.

Cheers

Phil Hobbs

Here it is in the CLY2, which is a mesfet:

https://dl.dropboxusercontent.com/u/53724080/Parts/Fets/CLY2_TQ.pdf

The negative slope corresponds to the high power dissipation region.
Maybe these are true non-pulsed DC measurements, so the effect could
be thermal. Just like some RF guys to do that.

I'm pretty sure I've seen the same in a phemt, but I can't find it
just now.

The DC specs are awful (in the sense of missing) on most RF parts. You
are lucky to get anything beyond Idss and drain breakdown voltage, and
you shouldn't believe the latter. The app notes literally say "bias it
until it works." We run supposedly 6 volt parts at 10, because they
actually start to leak at 25. That's RFthink again.


Hmm, interesting. It does look like a thermal effect, since the slope
changes sign as you go to higher dissipation. The top curve drops about
100 mA in 6 volts, which would be a drain impedance of -60 ohms. That
would be interesting to stabilize!

We mostly use these parts switchmode, so those curves don't much
matter.

Depletion gaasfets enhance past Idss, as much as 2:1 in some cases,
less for lazy mesfets and more for phemts with low pinchoff voltages.
The enhancement phemts, like the Avagos, have a weird
super-enhancement mode that kicks in with gate current, as much as you
dare to force.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[John Larkin]

On Wed, 8 Jul 2015 01:10:36 -0700 (PDT), Klaus Kragelund
<klauskvik@hotmail.com> wrote:

On Wednesday, July 8, 2015 at 12:35:41 AM UTC+2, John Larkin wrote:
On Tue, 07 Jul 2015 14:56:45 -0700, sms <scharf.steven@geemail.com
wrote:

On 7/7/2015 12:56 AM, John S wrote:
On 7/7/2015 2:48 AM, Klaus Kragelund wrote:
On Tuesday, July 7, 2015 at 2:54:30 AM UTC+2, krw wrote:
On Mon, 06 Jul 2015 16:23:07 -0700, sms <scharf.steven@geemail.com
wrote:

The place I'm contracting has one of these:
https://othermachine.co/othermill/features/> and they want me to
use it
to build some prototype boards.

They're going to bring over a Mac tomorrow since I don't have one to
use
with the machine (incredibly, there is no Windows software for this
machine).

Hoping it can do 0.05" spacing well enough.

There has been a tool something like this in the last two places I've
worked. Neither had/has been used in the time I've been there. By
the time you get the layout done, it's easier and better to just get a
board made. They're a waste, IMO.

The last place I worked we had a LKPD milling machine. We used it at
least 4-5 times per week and it was great for producing fast
prototyping. We could come up with an idea in the morning and have a
working PCB in the afternoon. With PCB prototyping that takes a week

Cheers

Klaus

What did you do about the PTH?

You can't do those. No big deal these days with so few through-hold parts.

The issue is vias.


Vias are pretty easy to do:

http://www.lpkf.com/products/rapid-pcb-prototyping/through-hole-plating/index.htm

Rivets! 1960s technology. Not very reliable, as I recall.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[Tim Williams]

"John Larkin" <jlarkin@highlandtechnology.com> wrote in message
news:u5d0qadhm0fcoffos9bek0st8pjn6ovu2r@4ax.com...

Depletion gaasfets enhance past Idss, as much as 2:1 in some cases,
less for lazy mesfets and more for phemts with low pinchoff voltages.
The enhancement phemts, like the Avagos, have a weird
super-enhancement mode that kicks in with gate current, as much as you
dare to force.

Should be a minority carrier thing, like a UJT. Supposedly, it happens to
regular Si JFETs as well (i.e., gate emission reducing channel resistance
below nominal Rds(on)), but I've never observed it.

On a related note, you can make a UJT out of a CdS photocell, LED and
resistor. Quite a bit slower and larger, but does the job (physically and
behaviorally).

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

 

[John Larkin]

On Fri, 10 Jul 2015 18:24:07 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

"John Larkin" <jlarkin@highlandtechnology.com> wrote in message
news:u5d0qadhm0fcoffos9bek0st8pjn6ovu2r@4ax.com...
Depletion gaasfets enhance past Idss, as much as 2:1 in some cases,
less for lazy mesfets and more for phemts with low pinchoff voltages.
The enhancement phemts, like the Avagos, have a weird
super-enhancement mode that kicks in with gate current, as much as you
dare to force.

Should be a minority carrier thing, like a UJT. Supposedly, it happens to
regular Si JFETs as well (i.e., gate emission reducing channel resistance
below nominal Rds(on)), but I've never observed it.

Seems to be something like that. Phemt gates are fragile (abs max gate
currents in the mA range) so you can't go crazy here.

On a related note, you can make a UJT out of a CdS photocell, LED and
resistor. Quite a bit slower and larger, but does the job (physically and
behaviorally).

Or a LASCR (light-activated SCR)


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[Tom Swift]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

On 07/10/2015 04:31 AM, Tom Swift wrote:
Phil Hobbs <hobbs@electrooptical.net> wrote:

SiGe:C transistors such as the BFP640 have effectively infinite
collector resistance, so cascoding pHEMTs with SiGe:C BJTs gives you
the best of all worlds: noise temperatures below 30 K and high,
linear gain.

Cheers

Phil Hobbs

Phil, here is a related question.

In the thread "Re: emitter follower gain", you posted the following:

A 40-GHz transistor with super low capacitance, high beta, and a
practically infinite V_A lets you do magical things, eg a cascoded
pHEMT front end with 0.3 nV 1-Hz noise, < 1 pF of capacitance, and
an RC-coupled voltage gain of 50 from DC to some ridiculous
frequency.

(The pHEMT has a 1/f corner of ~10 MHz, unfortunately.)

I posted a circuit like that a year or so back, in the "electrometer
front end" thread.

The post is http://tinyurl.com/pdw6nwu

I searched through the thread you referenced and could not find the
circuit. I searched google groups for all the LTspice circuits you
posted, and searched your web site, but could not find it.

Do you have a copy of the circuit and if so, could you post it again?

Thanks

The subthread about the circuit begins with a post whose time stamp is
06/05/2014 01:09 PM. On Google it's
https://groups.google.com/d/msg/sci.electronics.design/5nqKFRv_HnI/Ig9t
zZBKx-gJ

or

http://tinyurl.com/okwupty .

The actual circuit is slightly different, because the SPICE model
abuses the CFA output stage horribly, but this is the gist. It lives
and dies by keeping the input capacitance very very low, like
wire-bonding the biochip right to the board.

Worked great, and almost made the ridiculous spec.

Cheers

Phil Hobbs

Thanks very much for your troubles. That is a very elegant solution to a
difficult problem and I had already played with it a great deal. Truly
amazing performance!

However, it is a current amplifier. When you mentioned a voltage gain of
50, I thought you meant a voltage amplifier and that's why I could not
find a circuit. That was the reason for the question.

But the basic principle of driving a phemt in cascode with a bipolar
remains sound. I wonder if it ould be possible to adjust your circuit to
provide feedback to the source of the ATF38143 and turn it into a
wideband voltage amplifier? It might be worth investigating.

In the meantime, I may have found a solution for those who wish to upload
a LTspice file but who don't have their own web site or subscribe to
Dropbox.

http://www.tinyupload.com/ is free, extremely easy to use, and doesn't
require any kind of registration or giving them your email address or
credit card.

I fixed the line wrap in your circuit and uploaded it, and I wonder if
you could take a second and see if it works.

The circuit is at

http://s000.tinyupload.com/?file_id=10139622129014350173

Just click on 46EAA174.ASC and save it to a convenient folder.

The PLT file is at

http://s000.tinyupload.com/?file_id=40740846394056655321

Click on 46EAA174.PLT and save it to the same folder, then run LTspice.

The filenames are my local date and time converted to hex. This only
occurs once, so there is little risk these files will overwrite anything
on your hard disk.

This might solve the problem of line wrap wrecking LTspice uploads in a
newsgroup post.

Please let me know if it works and if it is convenient to use.

Thanks for all your help.

 

[Tom Swift]

Phil Hobbs <pcdhobbs@gmail.com> wrote:

I searched through the thread you referenced and could not find the
circuit. I searched google groups for all the LTspice circuits you
posted, and searched your web site, but could not find it.

Do you have a copy of the circuit and if so, could you post it again?

Don't havee it right handy, but it's near the end of the "electrometer
front end for DMM" thread from May '14.

Cheers

Phil Hobbs

Thanks, I replied to your next post.

 

[Tom Swift]

Phil Hobbs <hobbs@electrooptical.net> wrote:

On 7/10/2015 2:54 AM, Tom Swift wrote:
Phil Hobbs <hobbs@electrooptical.net> wrote:

It's simpler than that--drain impedance is just the reciprocal of
the drain admittance, i.e. 1/(dI_d/dV_ds).

Thanks. That is a big help.

In BJTs, collector impedance is Early voltage divided by collector
current. Normally that's a big number.

It needs to be high in order for a follower or current source to
have good performance.

You have mentioned this a few times. For example, in the thread "Re:
Very Low Power Preamp", you state:

"Besides the 1/f noise, the main down side of pHEMTs is that they
have pathetically low drain impedances, down in the hundreds of ohms.
That makes them useless for followers, for instance, and seriously
limits the voltage gain you can get out of them. Cascoding them
helps a lot."

Why does low drain impedance kill the follower performance?

Because it looks like a resistor from source to drain, which forms a
voltage divider with the transconductance and whatever is providing
the source current. That makes the gain quite a bit less than 1. It
also varies with V_DS, so the follower is intrinsically nonlinear,
again even if you have a stiff bias current.

They're very fast, though, and very quiet in the flatband (the
ATF38143's voltage noise is about 0.3 nV/sqrt(Hz), and its gate
current is a bit under a nanoamp, so its current noise is pretty low,
like 5-10 fA per root hertz. The other nice thing about them is that
they're surprisingly stable. It isn't that easy to make a pHEMT
oscillate, IME, whereas the SiGe:C transistors sing like little
gigahertz birdies at the slightest provocation. (They're three times
as fast, of course, and have higher gain.) I've successfully used
this technique in hand wired protos like this one

http://electrooptical.net/www/sed/photoreceiver.jpg ,

but you have to keep the collector current down below a milliamp.

The pHEMT is just above the ferrite bead on the wiper of the bias pot,
and the SiGe:C BJT is immediately to its left. All the 1/8-W
resistors and leaded caps and stuff are in the bias circuitry.

SiGe:C transistors such as the BFP640 have effectively infinite
collector resistance, so cascoding pHEMTs with SiGe:C BJTs gives you
the best of all worlds: noise temperatures below 30 K and high,
linear gain.

From your previous post:

The ATF38143 is so droopy that you can't even use it as a source
follower, even with a really stiff tail current source.

What do you mean by "droopy"? What does it do to the signal?

See above.

Cheers

Phil Hobbs

Thanks very much for your excellent explanation. I will have to see if
LTapice can duplicate the device curves for the ATF38143 and examine this
further.

Very good work! Thanks.

 

[Tom Swift]

Tom Swift <spam@me.com> wrote:

Thanks very much for your excellent explanation. I will have to see if
LTapice can duplicate the device curves for the ATF38143 and examine
this further.

I looked at the performance as an emitter follower and it didn't seem all
that bad to me. Some results are at

http://s000.tinyupload.com/?file_id=54130764029191346435

Download ATF3413.ZIP and unzip in your download folder. The contents are

46EB25D6.ASC ATF34143_chip Drain Curves

The curve at Vgs = 0 shows a drain resistance of (4.992 - 0.996) /
(140.2e-3 - 111.4e-3) = 138.75 ohms, which is a bit less than your 160
ohm figure.

One problem with probing wideband signals with a 500 ohm resistive probe
is the 20 dB loss of signal. If the signal is weak already, it may be
difficult to see in the noise. One solution is to se an active probe, but
these can be extremely expensive at microwave frequencies. I decided to
see if the ATF34143 might be useful as a source follower.

46EB29C7.ASC ATF34143_chip Source Follower @ 117mA

This shows the performance as a wideband source follower with VEE
adjusted for 0V DC offset. The input is 5V p-p or about 18 dBm. The
output is about 4.53V p-p, or about 17.1 dBm. This is a loss of around
0.9 dB, which could be similar to loss of a short length of miniature
coax at microwave frequencies connecting the probe to the receiver.

The ic dissipates 384 mW, which is well under the maximum of 725 mW, but
it will require heat sinking.

One way to do this is to dump the heat into the coax shield, similar to
the method used in "A Novel High Resolution E-Field Microscope", at

http://epubs.surrey.ac.uk/803508/1/ARFTG_2011_finalrev_tech_paper_mod.pdf

There doesn't seem to be any gate resistor to ground, so I'm assuming the
horrendous gate leakage goes directly to the source, giving the chip 0V
Vgs bias. The next plot shows how it might work as a common emitter
ampilfier:

46EB8BE5.ASC ATF34143_chip Common Emitter @ 118mA

The power is brought in on the center coax lead, and terminates in 50
ohms at the receiver. The input signal is 50 mV p-p, and the output is
478 mV, for a gain of 478 / 50 = 9.56 or 19.6 dB.

Not bad for an ic that costs $185 at Newark:

https://octopart.com/search?q=ATF34143

 

[John Larkin]

On Sun, 12 Jul 2015 00:48:30 GMT, Tom Swift <spam@me.com> wrote:

Tom Swift <spam@me.com> wrote:

Thanks very much for your excellent explanation. I will have to see if
LTapice can duplicate the device curves for the ATF38143 and examine
this further.

I looked at the performance as an emitter follower and it didn't seem all
that bad to me. Some results are at

http://s000.tinyupload.com/?file_id=54130764029191346435

Download ATF3413.ZIP and unzip in your download folder. The contents are

46EB25D6.ASC ATF34143_chip Drain Curves

The curve at Vgs = 0 shows a drain resistance of (4.992 - 0.996) /
(140.2e-3 - 111.4e-3) = 138.75 ohms, which is a bit less than your 160
ohm figure.

One problem with probing wideband signals with a 500 ohm resistive probe
is the 20 dB loss of signal. If the signal is weak already, it may be
difficult to see in the noise. One solution is to se an active probe, but
these can be extremely expensive at microwave frequencies. I decided to
see if the ATF34143 might be useful as a source follower.

46EB29C7.ASC ATF34143_chip Source Follower @ 117mA

This shows the performance as a wideband source follower with VEE
adjusted for 0V DC offset. The input is 5V p-p or about 18 dBm. The
output is about 4.53V p-p, or about 17.1 dBm. This is a loss of around
0.9 dB, which could be similar to loss of a short length of miniature
coax at microwave frequencies connecting the probe to the receiver.

The ic dissipates 384 mW, which is well under the maximum of 725 mW, but
it will require heat sinking.

One way to do this is to dump the heat into the coax shield, similar to
the method used in "A Novel High Resolution E-Field Microscope", at

http://epubs.surrey.ac.uk/803508/1/ARFTG_2011_finalrev_tech_paper_mod.pdf

Looks like both source pins are grounded.

As a follower, the ATF34143 would probably want to oscillate.


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

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

 

[Tom Swift]

John Larkin <jlarkin@highlandtechnology.com> wrote:

On Sun, 12 Jul 2015 00:48:30 GMT, Tom Swift <spam@me.com> wrote:

The ic dissipates 384 mW, which is well under the maximum of 725 mW,
but it will require heat sinking.

One way to do this is to dump the heat into the coax shield, similar
to the method used in "A Novel High Resolution E-Field Microscope", at

http://epubs.surrey.ac.uk/803508/1/ARFTG_2011_finalrev_tech_paper_mod.p
df

Looks like both source pins are grounded.

In that application, sure. It has to be grounded.

However, as a source follower you could solder the drain tab (where the
heat is generated) to a piece of copper of a suitable shape, then couple
the heat into the coax shield.

> As a follower, the ATF34143 would probably want to oscillate.

You have to expect it. Wideband circuits like to oscillate. There are
some standard tricks that might help - a small ferrite bead on the gate,
carefully bypassing the drain to ground, perhaps a bead on the source,
making sure the coax is properly terminated, generally try to keep things
resistive with very short leads, and so on.

There still might be some pathological source impedance that would send
it into oscillaton, like a small inductor of the right value. This might
require some more fiddling. Have to try it and see. But if you check the
price on active microwave probes it would justify a bit of time spent
gettng it to work.

For example, the price on a used 1134A InfiniiMax 7 GHz Probe is $7,318,
Qty 1 at

http://www.testequity.com/products/1775/#Price

And then, probing is risky business. How would you feel if you destroyed
it by mistake?

I'd feel a lot better destroying an ATF34143.

 

[Phil Hobbs]

On 7/11/2015 8:48 PM, Tom Swift wrote:

Tom Swift <spam@me.com> wrote:

Thanks very much for your excellent explanation. I will have to see if
LTapice can duplicate the device curves for the ATF38143 and examine
this further.

I looked at the performance as an emitter follower and it didn't seem all
that bad to me. Some results are at

http://s000.tinyupload.com/?file_id=54130764029191346435

That's just SPICE, though. Experimentally the follower performance of
an ATF38143 isn't as good as 0.9 gain. I don't think it follows the
standard MESFET model that well.

Download ATF3413.ZIP and unzip in your download folder. The contents are

46EB25D6.ASC ATF34143_chip Drain Curves

The curve at Vgs = 0 shows a drain resistance of (4.992 - 0.996) /
(140.2e-3 - 111.4e-3) = 138.75 ohms, which is a bit less than your 160
ohm figure.

One problem with probing wideband signals with a 500 ohm resistive probe
is the 20 dB loss of signal. If the signal is weak already, it may be
difficult to see in the noise. One solution is to se an active probe, but
these can be extremely expensive at microwave frequencies. I decided to
see if the ATF34143 might be useful as a source follower.

46EB29C7.ASC ATF34143_chip Source Follower @ 117mA

This shows the performance as a wideband source follower with VEE
adjusted for 0V DC offset. The input is 5V p-p or about 18 dBm. The
output is about 4.53V p-p, or about 17.1 dBm. This is a loss of around
0.9 dB, which could be similar to loss of a short length of miniature
coax at microwave frequencies connecting the probe to the receiver.

For that use, it's probably fine, but a follower with a gain of 0.9
isn't useful to me very often, even if the ATF38143 were really that
good. I'm usually interested in accurate followers, for instance in
bootstraps. Some of them have wideband gains of more than 0.999,
measured by the bandwidth improvement obtained in the bootstrap. That's
very difficult with a pHEMT, even with its drain cascoded or bootstrapped.

The ic dissipates 384 mW, which is well under the maximum of 725 mW, but
it will require heat sinking.

Are we talking about the same device? The ATF34143 is a single
transistor in a 4-lead SC70. Its max dissipation is listed as 725 mW,
*with the source lead at 25 C*. Since its thermal resistance theta_JC
is specified at 165 K/W (which is probably optimistic), making the
source lead 25C puts the junction at 0.58*165 + 25 = 145 C. Your 384
mW will cook the device in real life--90 C even with an infinite heat sink.

If you look at Fig. 1 of the datasheet, the spacing between the curves
varies by a good factor of 3, which is the nonlinearity I'm talking
about. It's pretty horrible.

One way to do this is to dump the heat into the coax shield, similar to
the method used in "A Novel High Resolution E-Field Microscope", at

http://epubs.surrey.ac.uk/803508/1/ARFTG_2011_finalrev_tech_paper_mod.pdf

There doesn't seem to be any gate resistor to ground, so I'm assuming the
horrendous gate leakage goes directly to the source, giving the chip 0V
Vgs bias.

Interestingly, that isn't the case in general. pHEMTs tend to self-bias
in weird ways--the SKY65050 will actually pull its gate negative
spontaneously, so that its zero gate current point is about -200 mV.
None of the models show this.

The next plot shows how it might work as a common emitter

ampilfier:

46EB8BE5.ASC ATF34143_chip Common Emitter @ 118mA

I used to do the same thing with the late lamented MRF966 dual-gate GaAs
MESFET. Being a cascode internally, it was a lot more linear. It also
worked well with zero volts G2-S, which was very convenient, and it came
in the old Motorola Macro X package, so I just used the gate lead as the
probe. Very useful.

The power is brought in on the center coax lead, and terminates in 50
ohms at the receiver. The input signal is 50 mV p-p, and the output is
478 mV, for a gain of 478 / 50 = 9.56 or 19.6 dB.

Not bad for an ic that costs $185 at Newark:

https://octopart.com/search?q=ATF34143

You're pessimistic by 2 orders of magnitude. The 38143 is a smaller
device that I pay about 50 cents for.

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

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

 

[Phil Hobbs]

On 7/11/2015 9:15 PM, John Larkin wrote:

On Sun, 12 Jul 2015 00:48:30 GMT, Tom Swift <spam@me.com> wrote:

Tom Swift <spam@me.com> wrote:

Thanks very much for your excellent explanation. I will have to see if
LTapice can duplicate the device curves for the ATF38143 and examine
this further.

I looked at the performance as an emitter follower and it didn't seem all
that bad to me. Some results are at

http://s000.tinyupload.com/?file_id=54130764029191346435

Download ATF3413.ZIP and unzip in your download folder. The contents are

46EB25D6.ASC ATF34143_chip Drain Curves

The curve at Vgs = 0 shows a drain resistance of (4.992 - 0.996) /
(140.2e-3 - 111.4e-3) = 138.75 ohms, which is a bit less than your 160
ohm figure.

One problem with probing wideband signals with a 500 ohm resistive probe
is the 20 dB loss of signal. If the signal is weak already, it may be
difficult to see in the noise. One solution is to se an active probe, but
these can be extremely expensive at microwave frequencies. I decided to
see if the ATF34143 might be useful as a source follower.

46EB29C7.ASC ATF34143_chip Source Follower @ 117mA

This shows the performance as a wideband source follower with VEE
adjusted for 0V DC offset. The input is 5V p-p or about 18 dBm. The
output is about 4.53V p-p, or about 17.1 dBm. This is a loss of around
0.9 dB, which could be similar to loss of a short length of miniature
coax at microwave frequencies connecting the probe to the receiver.

The ic dissipates 384 mW, which is well under the maximum of 725 mW, but
it will require heat sinking.

One way to do this is to dump the heat into the coax shield, similar to
the method used in "A Novel High Resolution E-Field Microscope", at

http://epubs.surrey.ac.uk/803508/1/ARFTG_2011_finalrev_tech_paper_mod.pdf

Looks like both source pins are grounded.

As a follower, the ATF34143 would probably want to oscillate.


The 38143 at 10 mA doesn't even need a bead in its gate. Magic.

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

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

 

[Phil Hobbs]

I was looking at the 38143 datasheet until I noticed you were mainly talking about the 34143, which has a lower max dissipation. I changed the result but missed the LHS. The calculation is otherwise correct.

I do build 50-ohm stuff, but it's not my main interest.

For FET probes, I use Tek P6249s from eBay (4 GHz, about $250). For slower stuff, i.e. with my 500-MHz scopes, I generally use P6201s (900 MHz).

Cheers

Phil Hobbs