I just invented the saturating cascode. I suppose 7000 other people
did it first, but that doesn't make it less interesting.

https://www.dropbox.com/s/p8xgwdqeuj4yty6/Sat_Cascode.jpg?raw=1

This would work with any sort of fet, maybe even bipolars. Maybe not
depletions. All four fets go ohmic when it's on. It's fast because
there is very little Miller effect on the bottom pair, and the top
ones get driven hard.

I have a cool circuit that's a bit wimpy on pulldown current. If I add
a second fet in parallel, or use a bigger part, it gets too slow. This
might fix that.

John Larkin wrote:
>
> I just invented the saturating cascode. I suppose 7000 other people
> did it first, but that doesn't make it less interesting.
>
> https://www.dropbox.com/s/p8xgwdqeuj4yty6/Sat_Cascode.jpg?raw=1
>
> This would work with any sort of fet, maybe even bipolars. Maybe not
> depletions. All four fets go ohmic when it's on. It's fast because
> there is very little Miller effect on the bottom pair, and the top
> ones get driven hard.
>
> I have a cool circuit that's a bit wimpy on pulldown current. If I add
> a second fet in parallel, or use a bigger part, it gets too slow. This
> might fix that.
>

Fun.

The falling edge would be vaguely fine with BJTs, but the rise would be
gross, on account of charge storage.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Mon, 28 Jun 2021 14:57:21 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>John Larkin wrote:
>>
>> I just invented the saturating cascode. I suppose 7000 other people
>> did it first, but that doesn't make it less interesting.
>>
>> https://www.dropbox.com/s/p8xgwdqeuj4yty6/Sat_Cascode.jpg?raw=1
>>
>> This would work with any sort of fet, maybe even bipolars. Maybe not
>> depletions. All four fets go ohmic when it's on. It's fast because
>> there is very little Miller effect on the bottom pair, and the top
>> ones get driven hard.
>>
>> I have a cool circuit that's a bit wimpy on pulldown current. If I add
>> a second fet in parallel, or use a bigger part, it gets too slow. This
>> might fix that.
>>
>
>Fun.
>
>The falling edge would be vaguely fine with BJTs, but the rise would be
>gross, on account of charge storage.
>
>Cheers
>
>Phil Hobbs

I'll be making a short negative pulse into a 50 ohm load, so the load
itself provides my pullup current.

Strangely, in my single-fet prototype, and in the simulated cascode,
the fall time is slower.

I intend to apply rigid scientific principles and analysis, namely lay
out a lot of proto boards and see if any work. Much below about 200
ps, I don't entirely trust Spice.

"Below 1 ns, things get hard. Below 100 ps, they get really hard."

John Larkin wrote:
> On Mon, 28 Jun 2021 14:57:21 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> John Larkin wrote:
>>>
>>> I just invented the saturating cascode. I suppose 7000 other people
>>> did it first, but that doesn't make it less interesting.
>>>
>>> https://www.dropbox.com/s/p8xgwdqeuj4yty6/Sat_Cascode.jpg?raw=1
>>>
>>> This would work with any sort of fet, maybe even bipolars. Maybe not
>>> depletions. All four fets go ohmic when it's on. It's fast because
>>> there is very little Miller effect on the bottom pair, and the top
>>> ones get driven hard.
>>>
>>> I have a cool circuit that's a bit wimpy on pulldown current. If I add
>>> a second fet in parallel, or use a bigger part, it gets too slow. This
>>> might fix that.
>>>
>>
>> Fun.
>>
>> The falling edge would be vaguely fine with BJTs, but the rise would be
>> gross, on account of charge storage.
>>
>> Cheers
>>
>> Phil Hobbs
>
> I'll be making a short negative pulse into a 50 ohm load, so the load
> itself provides my pullup current.
>
> Strangely, in my single-fet prototype, and in the simulated cascode,
> the fall time is slower.
>
> I intend to apply rigid scientific principles and analysis, namely lay
> out a lot of proto boards and see if any work. Much below about 200
> ps, I don't entirely trust Spice.
>
> "Below 1 ns, things get hard. Below 100 ps, they get really hard."
>
I stuck it into LTspice with that hacked-together SAV-551+ model, and
the spherical-cow showed about 80 ps fall time and 200 ps rise. That
was considerably faster than a plain common-source stage.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

John Larkin wrote:
> On Mon, 28 Jun 2021 14:57:21 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> John Larkin wrote:
>>>
>>> I just invented the saturating cascode. I suppose 7000 other people
>>> did it first, but that doesn't make it less interesting.
>>>
>>> https://www.dropbox.com/s/p8xgwdqeuj4yty6/Sat_Cascode.jpg?raw=1
>>>
>>> This would work with any sort of fet, maybe even bipolars. Maybe not
>>> depletions. All four fets go ohmic when it's on. It's fast because
>>> there is very little Miller effect on the bottom pair, and the top
>>> ones get driven hard.
>>>
>>> I have a cool circuit that's a bit wimpy on pulldown current. If I add
>>> a second fet in parallel, or use a bigger part, it gets too slow. This
>>> might fix that.
>>>
>>
>> Fun.
>>
>> The falling edge would be vaguely fine with BJTs, but the rise would be
>> gross, on account of charge storage.
>>
>> Cheers
>>
>> Phil Hobbs
>
> I'll be making a short negative pulse into a 50 ohm load, so the load
> itself provides my pullup current.
>
> Strangely, in my single-fet prototype, and in the simulated cascode,
> the fall time is slower.
>
> I intend to apply rigid scientific principles and analysis, namely lay
> out a lot of proto boards and see if any work. Much below about 200
> ps, I don't entirely trust Spice.
>
> "Below 1 ns, things get hard. Below 100 ps, they get really hard."
>

I stuck it into LTspice with that hacked-together SAV-551+ model, and
the spherical-cow showed about 80 ps fall time and 200 ps rise. That
was considerably faster than a plain common-source stage.

(Reposted to add model inline instead of a library.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Tue, 29 Jun 2021 11:24:24 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>John Larkin wrote:
>> On Mon, 28 Jun 2021 14:57:21 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>>> John Larkin wrote:
>>>>
>>>> I just invented the saturating cascode. I suppose 7000 other people
>>>> did it first, but that doesn't make it less interesting.
>>>>
>>>> https://www.dropbox.com/s/p8xgwdqeuj4yty6/Sat_Cascode.jpg?raw=1
>>>>
>>>> This would work with any sort of fet, maybe even bipolars. Maybe not
>>>> depletions. All four fets go ohmic when it's on. It's fast because
>>>> there is very little Miller effect on the bottom pair, and the top
>>>> ones get driven hard.
>>>>
>>>> I have a cool circuit that's a bit wimpy on pulldown current. If I add
>>>> a second fet in parallel, or use a bigger part, it gets too slow. This
>>>> might fix that.
>>>>
>>>
>>> Fun.
>>>
>>> The falling edge would be vaguely fine with BJTs, but the rise would be
>>> gross, on account of charge storage.
>>>
>>> Cheers
>>>
>>> Phil Hobbs
>>
>> I'll be making a short negative pulse into a 50 ohm load, so the load
>> itself provides my pullup current.
>>
>> Strangely, in my single-fet prototype, and in the simulated cascode,
>> the fall time is slower.
>>
>> I intend to apply rigid scientific principles and analysis, namely lay
>> out a lot of proto boards and see if any work. Much below about 200
>> ps, I don't entirely trust Spice.
>>
>> "Below 1 ns, things get hard. Below 100 ps, they get really hard."
>>
>
>I stuck it into LTspice with that hacked-together SAV-551+ model, and
>the spherical-cow showed about 80 ps fall time and 200 ps rise. That
>was considerably faster than a plain common-source stage.
>
>(Reposted to add model inline instead of a library.)
>
>Cheers
>
>Phil Hobbs

Cool. I thought this was a text group that didn't allow attachments.

In my version, I used different fets, 4 of them, and drove them
harder; ~1 volt gate drive from low impedance, and higher cascode
bias. My duty cycle will be very low (envision firing largish lasers)
so I'm optimistic I won't blow out the gates. I asssume that gross
gate over-drive makes them conduct harder; we'll see. [1]

I'm seeing pretty fast edges, but my fet model is crudely hacked from
yours, and doesn't include wirebond inductances.

https://www.dropbox.com/s/xxbz3dbtgaia1rx/Sat_Cascode_99.jpg?raw=1

(I am pretty heavily NDA'd on this one, so the concept is general. I
could go private if you are interested.)

[1] I have long suspected a bipolar gain mechanism in mesfets and
phemts, namely huge drain currents when the gate begins to conduct
serious current. Maybe that's just some exponential and some
imagination.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

jlarkin@highlandsniptechnology.com wrote:
> On Tue, 29 Jun 2021 11:24:24 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> John Larkin wrote:
>>> On Mon, 28 Jun 2021 14:57:21 -0400, Phil Hobbs
>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>
>>>> John Larkin wrote:
>>>>>
>>>>> I just invented the saturating cascode. I suppose 7000 other people
>>>>> did it first, but that doesn't make it less interesting.
>>>>>
>>>>> https://www.dropbox.com/s/p8xgwdqeuj4yty6/Sat_Cascode.jpg?raw=1
>>>>>
>>>>> This would work with any sort of fet, maybe even bipolars. Maybe not
>>>>> depletions. All four fets go ohmic when it's on. It's fast because
>>>>> there is very little Miller effect on the bottom pair, and the top
>>>>> ones get driven hard.
>>>>>
>>>>> I have a cool circuit that's a bit wimpy on pulldown current. If I add
>>>>> a second fet in parallel, or use a bigger part, it gets too slow. This
>>>>> might fix that.
>>>>>
>>>>
>>>> Fun.
>>>>
>>>> The falling edge would be vaguely fine with BJTs, but the rise would be
>>>> gross, on account of charge storage.
>>>>
>>>> Cheers
>>>>
>>>> Phil Hobbs
>>>
>>> I'll be making a short negative pulse into a 50 ohm load, so the load
>>> itself provides my pullup current.
>>>
>>> Strangely, in my single-fet prototype, and in the simulated cascode,
>>> the fall time is slower.
>>>
>>> I intend to apply rigid scientific principles and analysis, namely lay
>>> out a lot of proto boards and see if any work. Much below about 200
>>> ps, I don't entirely trust Spice.
>>>
>>> "Below 1 ns, things get hard. Below 100 ps, they get really hard."
>>>
>>
>> I stuck it into LTspice with that hacked-together SAV-551+ model, and
>> the spherical-cow showed about 80 ps fall time and 200 ps rise. That
>> was considerably faster than a plain common-source stage.
>>
>> (Reposted to add model inline instead of a library.)
>>
>> Cheers
>>
>> Phil Hobbs
>
> Cool. I thought this was a text group that didn't allow attachments.
>
> In my version, I used different fets, 4 of them, and drove them
> harder; ~1 volt gate drive from low impedance, and higher cascode
> bias. My duty cycle will be very low (envision firing largish lasers)
> so I'm optimistic I won't blow out the gates. I asssume that gross
> gate over-drive makes them conduct harder; we'll see. [1]
>
> I'm seeing pretty fast edges, but my fet model is crudely hacked from
> yours, and doesn't include wirebond inductances.
>
> https://www.dropbox.com/s/xxbz3dbtgaia1rx/Sat_Cascode_99.jpg?raw=1

Nice. Small inductances matter a lot--I recently talked about that
fancy vehicular lidar TX board that I got to blow up on purpose--the
same EPC GaNFETs you like, with a few-nanofarad 0402 cap and a
triple-stacked Osram diode laser, producing ~30A pulses of 5 ns FWHM. I
backed out the strays from my measurements, and found a really good fit
with 200 pH in the drain and 180 pH in the source. Not a bad layout!
(The source inductance was a worse problem.)

>
> (I am pretty heavily NDA'd on this one, so the concept is general. I
> could go private if you are interested.)
>
> [1] I have long suspected a bipolar gain mechanism in mesfets and
> phemts, namely huge drain currents when the gate begins to conduct
> serious current. Maybe that's just some exponential and some
> imagination.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On 29/6/21 4:41 am, John Larkin wrote:
>
> I just invented the saturating cascode. I suppose 7000 other people
> did it first, but that doesn't make it less interesting.
>
> https://www.dropbox.com/s/p8xgwdqeuj4yty6/Sat_Cascode.jpg?raw=1

Does this need four FETs, or just two? Did you add the extra pair for
stronger drive? Forgive me if I'm missing something...

Clifford Heath