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From: Paul Burridge
Subject: Re: Amplifying stage with *negative* Voltage gain??
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Date: Tue, 10 Dec 2002 14:06:29 +0000
NNTP-Posting-Date: Tue, 10 Dec 2002 15:34:51 GMT
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On Tue, 10 Dec 2002 08:53:07 -0800, Mike Monett opined
>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.
Confusing. Out of calibration makes the readings inaccurate; using the
X1 probe makes the readings inaccurate. Can't win! :-(
>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.
I do indeed, but curiously, the final reading ended up swinging
periodically up and down at around 9M every few seconds. Never seen
such behaviour from a passive device before. Unless it's something to
do with the action of the meter. I'm beginning to question the
God-like faith I had in measuring equipment. :-(
>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?
See my other reply to Tony.
>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.
I did try to pop out and borrow a sweep gen. earlier but my car locks
were frozen solid. That little test will have to wait...
>There might be a Nobel prize... - nah, probably not!
Only for sheer dogged determination! (on the part of all of us)
"I was going to help him up, but then I thumped him."
Michael Head's plea in mitigation to Weybridge Magistrates, Sept. 1975
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