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From: Spehro Pefhany on 25 Mar 2010 17:29

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?

From: Tim Wescott on 25 Mar 2010 17:36
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.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Joerg on 25 Mar 2010 17:45
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?
>

As Tim wrote, resistors. Usually one 0402 or smaller resistor around
50ohms (whatever you have there) right at the base and in series with it
suffices. A 0402 bypass cap from base to emitter also helps but you
could get away without one.

I don't know what you want to do with those. Not having the emitter on
ground can get very spooky. The collector can be separated by a snippet
of copper foil as a little "wall".

Most important is to breadboard on copperclad. I prefer living-bug style
with the devices glued to plastic spacer pieces. This snippets of FR4,
ice cram stick, wood shim pieces.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: Spehro Pefhany on 25 Mar 2010 17:52
On Thu, 25 Mar 2010 14:45:51 -0700, Joerg <invalid(a)invalid.invalid>
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?
>>
>
>As Tim wrote, resistors. Usually one 0402 or smaller resistor around
>50ohms (whatever you have there) right at the base and in series with it
>suffices.

Argh. Can't do that, 50R would ruin everything.

>A 0402 bypass cap from base to emitter also helps but you
>could get away without one.

Might have to go with that. There are very low values available.

>I don't know what you want to do with those. Not having the emitter on
>ground can get very spooky. The collector can be separated by a snippet
>of copper foil as a little "wall".
>
>Most important is to breadboard on copperclad. I prefer living-bug style
>with the devices glued to plastic spacer pieces. This snippets of FR4,
>ice cram stick, wood shim pieces.

Thanks, Joerg, Tim.

From: Joerg on 25 Mar 2010 18:26
Spehro Pefhany wrote:
> On Thu, 25 Mar 2010 14:45:51 -0700, Joerg <invalid(a)invalid.invalid>
> 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?
>>>
>> As Tim wrote, resistors. Usually one 0402 or smaller resistor around
>> 50ohms (whatever you have there) right at the base and in series with it
>> suffices.
>
> Argh. Can't do that, 50R would ruin everything.
>

Ok, how about a very small SMT ferrite bead? Not sure if a real bead
with a wire through it will work. It might if you drill a hole right in
front of the base and sort of sink the bead 1/3rd into the board,
lengthwise. Make sure the ferrite doesn't touch the ground plane anywhere.


>> A 0402 bypass cap from base to emitter also helps but you
>> could get away without one.
>
> Might have to go with that. There are very low values available.
>

You only need a few hundred pF and it doesn't have to be NP0.


>> I don't know what you want to do with those. Not having the emitter on
>> ground can get very spooky. The collector can be separated by a snippet
>> of copper foil as a little "wall".
>>
>> Most important is to breadboard on copperclad. I prefer living-bug style
>> with the devices glued to plastic spacer pieces. This snippets of FR4,
>> ice cram stick, wood shim pieces.
>
> Thanks, Joerg, Tim.
>

We are here to serve :-)

Thanks for the hint about the X7S datasheet in the other thread, made my
life easier. Drops the size of the caps from 2220 to 1812, meaning we
can get more on the board.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
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