The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Amplifying stage with *negative* Voltage gain??
References: <email@example.com> <3DEF1DC0.5EDD@Spam.Bots> <firstname.lastname@example.org> <3DEF5BEA.2614@Spam.Bots> <3DF0A422.46EE@Spam.Bots> <email@example.com> <3DF263CE.3868@Spam.Bots> <firstname.lastname@example.org> <email@example.com>
Date: Tue, 10 Dec 2002 08:53:07 -0800
NNTP-Posting-Date: Tue, 10 Dec 2002 08:53:10 EST
Organization: Bell Sympatico
Paul Burridge wrote:
> On Tue, 10 Dec 2002 12:20:07 +0000 (GMT), Tony Williams
> opined thusly:
> > Which is still pointing to more load on the output
> > 1nF than you have so far discovered. It could be a
> > capacitance, or even something more absurd... any
> > chance that the scope is stuck on 50-ohm input-R?
> I've just checked on the scope input itself and it says "1M Ohm [in
> parallel with] 30pf" earlier someone said it should be nearer 10M.,
> but this is *definitely* just 1M (as opposed to the probe itself which
> is higher - certainly on X10).
The standard scope input is 1 meg in parallel with 30 pf or so. You can
measure it with a dvm to verify, but it is not likely to be off by much.
All your readings would be out of calibration with a 10X probe.
You can also measure the resistance of the probe. On the 1X setting, the
resistance should be low (short-circuit). On the 10X setting, you should
see 9 megs.
> The only other source of capacitance would be two tracks of copper
> 2.5" long on the stripboard I've used to build this thing. They're
> certainly in parallel with the 1n, but since we're only talking of the
> low megahertz here, I wouldn't have thought that would be a problem.
As usual, Tony is pointing us in the right direction. The symptoms
indicate some large capacitive load on the output. The trace capacitance
will be in the pf range, not enough to affect things. Can you remove the
1 nF output cap and measure directly at the pulldown resistor?
Also, using a lower frequency might help in debugging. If the waveform is
clipped at 1 KHz, there is definitely some new physical process going on
considering the voltage measurements you posted previously.
There might be a Nobel prize... - nah, probably not!
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup