Joerg
Guest
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 Posted:
Mon Nov 07, 2005 5:35 pm Post subject:
Re: Need replacement for BC847S better suited for HF |
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Hello Erik,
| Quote: | Just be careful with these. A GHz transistor can oscillate without you
even seeing it because none of the equipment is fast enough for that.
Do you really think, it will be a problem in my configuration?
The overall gain of my circuit is unity and at the 10mA of
DC current used, ft is 2G in tha case of the BFS17 and max.
8 in the case of the BFS482. And gain of both tranistors is
with around 13dB comparable, too?
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I have seen unity gain buffers oscillate. That was because some traces
weren't really shorts at high frequencies and the buffer developed some
'gain'.
| Quote: | I could design two versiobs of the board (one with BFS482s and
a second with lots of BFS17) but I think temperature drift is much
worser with the BFS17-solution. And this drift of DC-level
during warm up is the only "feature", which is inconvenient with
the russian oscilloscope.
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I'd go with the BFS482 if it is better. We have a hard time getting this
stuff in the US. EU manufacturers aren't that great with engineering
samples. Usually you can't get them and so we often don't use their
parts. Temp drift can often be fixed with a tracking diode.
Regards, Joerg
http://www.analogconsultants.com
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Erik Baigar
Guest
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| Posted:
Mon Nov 07, 2005 5:35 pm Post subject:
Re: Need replacement for BC847S better suited for HF |
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Joerg wrote:
| Quote: |
Hello Erik,
I think the BFS480 are "hotter" than the BFS17 - at least a factor
of two. The oscilloscope under investigation is a C1-122 and the
other componentes have -3dB at around 150MHz so a drop at 750MHz
(as I expect in using the BFS48x) might be OK. In my opinon the SOT363
package is the smallest I want to solder by hand ;-)
Just be careful with these. A GHz transistor can oscillate without you
even seeing it because none of the equipment is fast enough for that.
|
Do you really think, it will be a problem in my configuration?
The overall gain of my circuit is unity and at the 10mA of
DC current used, ft is 2G in tha case of the BFS17 and max.
8 in the case of the BFS482. And gain of both tranistors is
with around 13dB comparable, too?
I could design two versiobs of the board (one with BFS482s and
a second with lots of BFS17) but I think temperature drift is much
worser with the BFS17-solution. And this drift of DC-level
during warm up is the only "feature", which is inconvenient with
the russian oscilloscope.
Best regards,
Erik. |
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