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From: John Larkin on 25 Mar 2010 18:37
On Thu, 25 Mar 2010 17:29:19 -0400, Spehro Pefhany
<speffSNIP(a)interlogDOTyou.knowwhat> wrote:

>
>I'm looking at playing with some SiGe parts for a close-to-DC
>application (< 1MHz). These things have scary high fT like 30-80 GHz.
>
>Any tips for avoiding oscillation that I might not even be able to
>see? Ground plane, run at low current?

What do you intend to do with these things?

I breadboarded some of the Infineon 45 GHz SiGe things, soldered down
to copperclad. I was exploring how fast they would turn on as fairly
low current switches. They seemed stable but were surprisingly slow.

PHEMTS are faster in real life. Their feedback capacitance is absurdly
low so they tend to be stable. I have breadboarded phemt circuits on
copperclad and got good, fast, clean switching.

EL07 driving an NE3508:

ftp://jjlarkin.lmi.net/BB_fast.JPG

This gave me a pretty good 5-volt p-p square wave at 1 GHz.

John

From: Phil Hobbs on 25 Mar 2010 20:46
On 3/25/2010 5:36 PM, Tim Wescott wrote:
> Spehro Pefhany wrote:
>> I'm looking at playing with some SiGe parts for a close-to-DC
>> application (< 1MHz). These things have scary high fT like 30-80 GHz.
>> Any tips for avoiding oscillation that I might not even be able to
>> see? Ground plane, run at low current?
> Do some of the indirect tests for oscillation that amateur radio
> operators have learned to do...
>
> You may be able to put Q-killing resistors in the relevant leads -- the
> idea is to put low-value resistors up close, to absorb energy in the
> return path.
>
> It will, of course, knock your desired properties a bit, but if you've
> got the headroom you may as well buy some stability margin with it.
>
> If I didn't do that, I'd want to analyze the layout with an eye toward
> transmission line behavior, if not flat design a little amplifier cell
> with all the right loading to be unconditionally stable.
>
> To some extent you'll be gaining stability by using 'ordinary' FR-4, but
> that just means that some day a board house's 'improvement' will knock
> your circuit for a loop.
>

I'd be looking at beads, personally--some of those nice SiGe NPNs have
1-Hz voltage noise down at the 200 pV level, which is the Johnson noise
of a 0.5 ohm resistor. The beads' noise doesn't really kick in till
they start to look resistive.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: MooseFET on 25 Mar 2010 20:51
On Mar 25, 2:29 pm, Spehro Pefhany <speffS...(a)interlogDOTyou.knowwhat>
wrote:
> I'm looking at playing with some SiGe parts for a close-to-DC
> application (< 1MHz). These things have scary high fT like 30-80 GHz.
>
> Any tips for avoiding oscillation that I might not even be able to
> see? Ground plane, run at low current?

The ground plane may help.

Lossy things right at the legs may also help.

Low bias currents will help.

Lowish RF impedances will help.
From: Spehro Pefhany on 26 Mar 2010 00:24
On Thu, 25 Mar 2010 15:37:50 -0700, the renowned John Larkin
<jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Thu, 25 Mar 2010 17:29:19 -0400, Spehro Pefhany
><speffSNIP(a)interlogDOTyou.knowwhat> wrote:
>
>>
>>I'm looking at playing with some SiGe parts for a close-to-DC
>>application (< 1MHz). These things have scary high fT like 30-80 GHz.
>>
>>Any tips for avoiding oscillation that I might not even be able to
>>see? Ground plane, run at low current?
>
>What do you intend to do with these things?

Amplify some relatively low frequency sinusoidal signals. Those parts
can have some interesting noise characteristics.

The ferrite bead sounds like a good idea... hmm might have to work
with the &*&$&*$ microscopic 0201 parts to get much Z at a few GHz.

>I breadboarded some of the Infineon 45 GHz SiGe things, soldered down
>to copperclad. I was exploring how fast they would turn on as fairly
>low current switches. They seemed stable but were surprisingly slow.
>
>PHEMTS are faster in real life. Their feedback capacitance is absurdly
>low so they tend to be stable. I have breadboarded phemt circuits on
>copperclad and got good, fast, clean switching.
>
>EL07 driving an NE3508:
>
>ftp://jjlarkin.lmi.net/BB_fast.JPG
>
>This gave me a pretty good 5-volt p-p square wave at 1 GHz.
>
>John

Square wave @ 1Ghz. Ha.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff(a)interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
From: John Larkin on 25 Mar 2010 23:52
On Thu, 25 Mar 2010 23:24:12 -0500, Spehro Pefhany
<speffSNIP(a)interlogDOTyou.knowwhat> wrote:

>On Thu, 25 Mar 2010 15:37:50 -0700, the renowned John Larkin
><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:
>
>>On Thu, 25 Mar 2010 17:29:19 -0400, Spehro Pefhany
>><speffSNIP(a)interlogDOTyou.knowwhat> wrote:
>>
>>>
>>>I'm looking at playing with some SiGe parts for a close-to-DC
>>>application (< 1MHz). These things have scary high fT like 30-80 GHz.
>>>
>>>Any tips for avoiding oscillation that I might not even be able to
>>>see? Ground plane, run at low current?
>>
>>What do you intend to do with these things?
>
>Amplify some relatively low frequency sinusoidal signals. Those parts
>can have some interesting noise characteristics.

Oh. OK. I'd sort of come to expect RF parts to be leaky and noisy at
low frequencies, but that's apparently not the case, for bipolars at
least. A BFT25 c-b junction makes an incredible diode.

>
>The ferrite bead sounds like a good idea... hmm might have to work
>with the &*&$&*$ microscopic 0201 parts to get much Z at a few GHz.

I'd expect an 0603 to be OK. Like maybe 100 ohms at 100 MHz. Get it
working and touch a few nearby points with a pencil. If you don't see
any bias shifts, it's not oscillating.

>Square wave @ 1Ghz. Ha.

How about this:

http://www.highlandtechnology.com/DSS/T760DS.html

100 volt pulses out into 50 ohms. The latest spin has Tr and Tf both
under 1 ns.

John

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