Stabilizing pHEMTs

So, JL's thread about level shifting a pHEMT switch brings up another
issue: stabilizing the silly things.

I have this little SKY65050 preamp that works very well--about a nanoamp
of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin,
flat from DC to about 120 MHz where the THS3091 second stage craps out. 
A very nice amplifier, but for the intended use, it could stand a bit
less noise and a bit less input capacitance.  So I tried it with an
NE3508 instead, but I couldn't get it to stop oscillating.  

The pHEMT source is grounded, and its drain goes to the emitter of a
BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead. 
(This bead has high impedance way out beyond 1 GHz--otherwise the NPN
wants to oscillate at around 8 GHz.)

In actual use, the gate will be driven from some high impedance, so it
isn't instantly clear that a bead will help there.  Any experience with
stabilizing such a device?

Thanks

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

On Fri, 09 Mar 2012 14:05:00 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>So, JL's thread about level shifting a pHEMT switch brings up another
>issue: stabilizing the silly things.
>
>I have this little SKY65050 preamp that works very well--about a nanoamp
>of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin,
>flat from DC to about 120 MHz where the THS3091 second stage craps out. 
>A very nice amplifier, but for the intended use, it could stand a bit
>less noise and a bit less input capacitance.  So I tried it with an
>NE3508 instead, but I couldn't get it to stop oscillating.  
>
>The pHEMT source is grounded, and its drain goes to the emitter of a
>BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead. 
>(This bead has high impedance way out beyond 1 GHz--otherwise the NPN
>wants to oscillate at around 8 GHz.)
>
>In actual use, the gate will be driven from some high impedance, so it
>isn't instantly clear that a bead will help there.  Any experience with
>stabilizing such a device?
>
>Thanks
>
>Phil Hobbs

Hi, Phil,

I usually use them in switch mode, with a hard gate drive, and they
are OK there. If they did oscillate, I'd know it... jitter would go
way up.

The RF boys drive them with tuned circuits, matching the gate
impedance, and get that to work somehow.

So maybe the key is the gate impedance, which in your case is high.
You can't do anything about that, since that's important to your app.

Some microscopic bead in the gate might help. At these speeds, there
are no high impedances.

Can you do something to the drain, like add a bit of capacitance or
R-C to ground? Your signal BW is 120 MHz, so there is some room to
kill GHz gain. Just the wire bonds of the cascode NPN may be enough to
resonate.

PCB layout is important, too. You've got to really ground the source
leads, like to a topside pour with a lot of vias to ground. And
obviously keep the gate and drain from coupling. Maybe via the drain
to an inner layer ASAP.

I have used small mesfets and phemts in linear modes, but the gate and
drain impedances were low. I had to give that up not because of
oscillations, but because the gate DC offsets changed erratically,
trapping state migration or something like that.


-- 

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

Phil Hobbs wrote:
> So, JL's thread about level shifting a pHEMT switch brings up another
> issue: stabilizing the silly things.
>
> I have this little SKY65050 preamp that works very well--about a nanoamp
> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin,
> flat from DC to about 120 MHz where the THS3091 second stage craps out.
> A very nice amplifier, but for the intended use, it could stand a bit
> less noise and a bit less input capacitance.  So I tried it with an
> NE3508 instead, but I couldn't get it to stop oscillating.
>
> The pHEMT source is grounded, and its drain goes to the emitter of a
> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead.
> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN
> wants to oscillate at around 8 GHz.)
>
> In actual use, the gate will be driven from some high impedance, so it
> isn't instantly clear that a bead will help there.  Any experience with
> stabilizing such a device?
>
> Thanks
>
> Phil Hobbs
>
>
   Never tried, but heard a story where the PCB trace from driver to 
gate has a slab of ferrite on top, adjusted to the complex conjugate of 
the gate impedance.
   Worked like a champ but each one had to be tweaked - a PITA.

Robert Baer wrote:
> Phil Hobbs wrote:
>> So, JL's thread about level shifting a pHEMT switch brings up another
>> issue: stabilizing the silly things.
>>
>> I have this little SKY65050 preamp that works very well--about a nanoamp
>> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin,
>> flat from DC to about 120 MHz where the THS3091 second stage craps out.
>> A very nice amplifier, but for the intended use, it could stand a bit
>> less noise and a bit less input capacitance.  So I tried it with an
>> NE3508 instead, but I couldn't get it to stop oscillating.
>>
>> The pHEMT source is grounded, and its drain goes to the emitter of a
>> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead.
>> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN
>> wants to oscillate at around 8 GHz.)
>>
>> In actual use, the gate will be driven from some high impedance, so it
>> isn't instantly clear that a bead will help there.  Any experience with
>> stabilizing such a device?
>>
>> Thanks
>>
>> Phil Hobbs
>>
>>
>   Never tried, but heard a story where the PCB trace from driver to gate
> has a slab of ferrite on top, adjusted to the complex conjugate of the
> gate impedance.
>   Worked like a champ but each one had to be tweaked - a PITA.
> 

That's one of the tricks, rocks glued onto the board. One method is to
find the minimum size ferrite "rock" and then use a bigger one for
margin. Of course the glue is a bit of a concern, they shouldn't fall
off after some years.

Nastier: A 5ohm SMT bead (small), cutting disk in a Dremel, whirrrrrr
... cut until signal bandwidth begins to look good. Cut some more until
oscillation sets in. If those two points are far enough apart pick
something in the middle. There'll probably be enviro and OSHA issues to
deal with, dust control and all that.

-- 
Regards, Joerg

http://www.analogconsultants.com/

Phil Hobbs wrote:
> So, JL's thread about level shifting a pHEMT switch brings up another
> issue: stabilizing the silly things.
> 
> I have this little SKY65050 preamp that works very well--about a nanoamp
> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin,
> flat from DC to about 120 MHz where the THS3091 second stage craps out. 
> A very nice amplifier, but for the intended use, it could stand a bit
> less noise and a bit less input capacitance.  So I tried it with an
> NE3508 instead, but I couldn't get it to stop oscillating.  
> 
> The pHEMT source is grounded, and its drain goes to the emitter of a
> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead. 
> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN
> wants to oscillate at around 8 GHz.)
> 
> In actual use, the gate will be driven from some high impedance, so it
> isn't instantly clear that a bead will help there.  Any experience with
> stabilizing such a device?
> 

Question: Did you run the trace to the gate at the other side of the
ground/supply planes or sandwiched in?

A photo and maybe the layout would help.

-- 
Regards, Joerg

http://www.analogconsultants.com/

On 03/09/2012 08:05 PM, Phil Hobbs wrote:

> The pHEMT source is grounded, and its drain goes to the emitter of a
> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead.
> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN
> wants to oscillate at around 8 GHz.)
>
> In actual use, the gate will be driven from some high impedance, so it
> isn't instantly clear that a bead will help there.  Any experience with
> stabilizing such a device?

I have bought these avago phemts mentioned in the other thread to build
high-IP3 lo noise preamps for the 144 and 432 MHz ham bands. When I had
the transistors I simulated them and found that I couldn't do much
useful at these low frequencies. The gain at 100 MHz is outrageous and
the s-param stability criteria are depressing.

Things that can be done (with other GaAsFets, too) is a 20R in the
drain, which you probably won't like ( or you would not need the
cascode) and a _few_ mm of source leads. That helps at low frequencies
but will lead to oscillations at 10 GHz or so if driven too far.

Somehow this shifts the problem to higher frequencies, but if one
doesn't need 10 GHz performance one can design the circuit as a SHF
swamp.

You may also encounter waveguide oscillations that only show up if you
close the box that contains the amp. Absorber foam helps, as does
keeping the waveguide diameter small enough. That can be hard.

/Gerhard

On Friday, March 9, 2012 2:05:00 PM UTC-5, Phil Hobbs wrote:
> So, JL's thread about level shifting a pHEMT switch brings up another
> issue: stabilizing the silly things.
> 
> I have this little SKY65050 preamp that works very well--about a nanoamp
> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin,
> flat from DC to about 120 MHz where the THS3091 second stage craps out. 
> A very nice amplifier, but for the intended use, it could stand a bit
> less noise and a bit less input capacitance.  So I tried it with an
> NE3508 instead, but I couldn't get it to stop oscillating.  
> 
> The pHEMT source is grounded, and its drain goes to the emitter of a
> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead. 
> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN
> wants to oscillate at around 8 GHz.)
> 
> In actual use, the gate will be driven from some high impedance, so it
> isn't instantly clear that a bead will help there.  Any experience with
> stabilizing such a device?

In my limited experience, a big part of the oscillation problem
was from actual, physical feedback, such as coupling from lead-
to-lead.  That interacts with device parameters, e.g. capacitance
and bond-wire inductances, to make an oscillator.

I made an LNA with an NE251, a dual-gate GaAsFET (now obsolete)
that was absolutely gorgeous, feedback-wise.  That's a cascode
in a package, and it acted like it.

So, that's a long intro to this idea: if your cascode isn't
stiff, the pHEMT drain moves, and feeds back to the gate.
That's especially bad when the gate drive's hi-z.

Maybe that f.b. on the BJT's base isn't the answer.  I'm not
sure what is, but I'd try a low-value resistor for starters.
All you need is a dissipative element to kill the Q of the
BJT resonances.  10 ohms just might do.

The usual r.f. way to stabilize the FET for broadband service
would be to neutralize it.

-- 
Cheers,
James Arthur

On 03/09/2012 05:34 PM, Joerg wrote:
> Phil Hobbs wrote:
>> So, JL's thread about level shifting a pHEMT switch brings up another
>> issue: stabilizing the silly things.
>>
>> I have this little SKY65050 preamp that works very well--about a nanoamp
>> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin,
>> flat from DC to about 120 MHz where the THS3091 second stage craps out.
>> A very nice amplifier, but for the intended use, it could stand a bit
>> less noise and a bit less input capacitance.  So I tried it with an
>> NE3508 instead, but I couldn't get it to stop oscillating.
>>
>> The pHEMT source is grounded, and its drain goes to the emitter of a
>> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead.
>> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN
>> wants to oscillate at around 8 GHz.)
>>
>> In actual use, the gate will be driven from some high impedance, so it
>> isn't instantly clear that a bead will help there.  Any experience with
>> stabilizing such a device?
>>
>
> Question: Did you run the trace to the gate at the other side of the
> ground/supply planes or sandwiched in?
>
> A photo and maybe the layout would help.
>

See http://electrooptical.net/www/sed/pHEMT_probe/pHEMT_probe.html

There are a few tweaks: The zero-ohm resistor is actually a 1 uF cap, 
replaced with 1 pF for input capacitance measurements.  the pHEMT is a 
SKY65050 or NE3508, and the collector load is 200 ohms.  Supplies are 
about +-10 V.

Thanks

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

Phil Hobbs wrote:
> On 03/09/2012 05:34 PM, Joerg wrote:
>> Phil Hobbs wrote:
>>> So, JL's thread about level shifting a pHEMT switch brings up another
>>> issue: stabilizing the silly things.
>>>
>>> I have this little SKY65050 preamp that works very well--about a nanoamp
>>> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin,
>>> flat from DC to about 120 MHz where the THS3091 second stage craps out.
>>> A very nice amplifier, but for the intended use, it could stand a bit
>>> less noise and a bit less input capacitance.  So I tried it with an
>>> NE3508 instead, but I couldn't get it to stop oscillating.
>>>
>>> The pHEMT source is grounded, and its drain goes to the emitter of a
>>> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead.
>>> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN
>>> wants to oscillate at around 8 GHz.)
>>>
>>> In actual use, the gate will be driven from some high impedance, so it
>>> isn't instantly clear that a bead will help there.  Any experience with
>>> stabilizing such a device?
>>>
>>
>> Question: Did you run the trace to the gate at the other side of the
>> ground/supply planes or sandwiched in?
>>
>> A photo and maybe the layout would help.
>>
> 
> See http://electrooptical.net/www/sed/pHEMT_probe/pHEMT_probe.html
> 
> There are a few tweaks: The zero-ohm resistor is actually a 1 uF cap,
> replaced with 1 pF for input capacitance measurements.  the pHEMT is a
> SKY65050 or NE3508, and the collector load is 200 ohms.  Supplies are
> about +-10 V.
> 

The photo is a bit fuzzy in the upper left but it looks like the input
and output traces are all on the top layer. Input and output should be
on opposite sides of a plane. This doesn't have to be a ground plane but
should not be a split one. I can see some sort of split (faint dark lines).

Can't see C5, looks like it may be a bit far away. Which is a concern.
Generally, I'd always have a 0.1uF 0603 (or smaller size) for bypassing.
And smack dab at the top of R2. Anyhow, that trace from Q2 to R2 and
then on to U1 is a bit long and too close to the input trace (R11).
Moving U1 north by about 0.500" would clean that up quite well.

The via for C1 is too far from it, looks like more than 0.100".

A little ferrite rock in front of Q1's gate would help, right at Q1. But
it must be on the other side. Looks like you can afford a roll-off since
the THS3091 isn't really a rocket, rolling off around 200MHz. The bypass
cap C4 is too far away from it.

Please don't take this as dissing, just some ideas how to make the
layout a little better :-)

-- 
Regards, Joerg

http://www.analogconsultants.com/

On 03/10/2012 03:21 PM, Joerg wrote:
> Phil Hobbs wrote:
>> On 03/09/2012 05:34 PM, Joerg wrote:
>>> Phil Hobbs wrote:
>>>> So, JL's thread about level shifting a pHEMT switch brings up another
>>>> issue: stabilizing the silly things.
>>>>
>>>> I have this little SKY65050 preamp that works very well--about a nanoamp
>>>> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin,
>>>> flat from DC to about 120 MHz where the THS3091 second stage craps out.
>>>> A very nice amplifier, but for the intended use, it could stand a bit
>>>> less noise and a bit less input capacitance.  So I tried it with an
>>>> NE3508 instead, but I couldn't get it to stop oscillating.
>>>>
>>>> The pHEMT source is grounded, and its drain goes to the emitter of a
>>>> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead.
>>>> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN
>>>> wants to oscillate at around 8 GHz.)
>>>>
>>>> In actual use, the gate will be driven from some high impedance, so it
>>>> isn't instantly clear that a bead will help there.  Any experience with
>>>> stabilizing such a device?
>>>>
>>>
>>> Question: Did you run the trace to the gate at the other side of the
>>> ground/supply planes or sandwiched in?
>>>
>>> A photo and maybe the layout would help.
>>>
>>
>> See http://electrooptical.net/www/sed/pHEMT_probe/pHEMT_probe.html
>>
>> There are a few tweaks: The zero-ohm resistor is actually a 1 uF cap,
>> replaced with 1 pF for input capacitance measurements.  the pHEMT is a
>> SKY65050 or NE3508, and the collector load is 200 ohms.  Supplies are
>> about +-10 V.
>>
>
> The photo is a bit fuzzy in the upper left but it looks like the input
> and output traces are all on the top layer. Input and output should be
> on opposite sides of a plane. This doesn't have to be a ground plane but
> should not be a split one. I can see some sort of split (faint dark lines).
>
> Can't see C5, looks like it may be a bit far away. Which is a concern.
> Generally, I'd always have a 0.1uF 0603 (or smaller size) for bypassing.
> And smack dab at the top of R2. Anyhow, that trace from Q2 to R2 and
> then on to U1 is a bit long and too close to the input trace (R11).
> Moving U1 north by about 0.500" would clean that up quite well.
>
> The via for C1 is too far from it, looks like more than 0.100".
>
> A little ferrite rock in front of Q1's gate would help, right at Q1. But
> it must be on the other side. Looks like you can afford a roll-off since
> the THS3091 isn't really a rocket, rolling off around 200MHz. The bypass
> cap C4 is too far away from it.
>
> Please don't take this as dissing, just some ideas how to make the
> layout a little better :-)
>
Thanks.

No worries about dissing me about the layout--If it's wrong, Mother 
Nature will point it out a lot less gently than you.

The via near C1 isn't the ground side, it's the R9 side.  C1, C5, and Q1 
all have ground vias right inside their pads.  There's a ground cutout 
under the pHEMT to reduce the pad capacitances, with the source leads 
grounded on either corner of the cutout.  (There's a zipfile with the 
board layout in PDF and Gerber.)

I'm not sure that it's input-output related, because it's lovely and 
stable with a SKY65050, which theoretically has about the same f_max as 
the NE3508, but I could certainly be mistaken about that.

C4 is only about 0.1 inch from the THS3091 (see the C-grid header for 
scale).  Is that really too far for a 200 MHz amp?  It should be only 
about 2 nH or so.

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