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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Amplifying stage with *negative* Voltage gain??
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Date: Tue, 10 Dec 2002 04:28:42 -0800
NNTP-Posting-Date: Tue, 10 Dec 2002 04:28:44 EST
Organization: Bell Sympatico
Paul Burridge wrote:
> On Sat, 07 Dec 2002 13:10:38 -0800, Mike Monett opined
> >When the probe is set to X1, the resistor and cap are shorted. The probe
> >cable and input capacity of the scope now appear at the probe tip. This
> >loads the circuit, and is what Tony referred to.
> I see. So what's X1 any good for then? Just low frequency audio
Basically, yes unless you have an unusually low source impedance.
> My output signal does appear to be quite markedly clipped at the
> bottom edge, but I can't see how current starvation should be a
> problem since I reduced RE to 400 ohms. There's a pretty healthy level
> of Ie running now and this config is *supposed* to be very low-z out
> and thus capable of driving such a modest load as a scope!
Should be no problem unless there is another hidden capacitor connected to the
output, or other problems. I describe some below.
> >I simulated this effect using load capacitances from 0pf to 400pf:
> > http://www3.sympatico.ca/add.automation/misc/cl400.gif
> >There are 5 different waveforms superimposed. They represent the value of
> >the cable and scope capacitance in 100pf steps. Do any of these waveforms
> >resemble what you see on the scope?
> Yes, flattened off at the bottom.
Hah! Now we are getting somewhere.
You mentioned in another post you switched to a tek 10X probe and got the same
results. If there is no additional capacitor connected to the output, this seems
impossible to understand.
The clipping at the bottom is normally an indication of excessive capacitive
loading, insufficient pulldown current, a bad or mismarked component, or perhaps
a bad transistor.
It it possible to use a much lower source frequency to help debug by removing the
capacitive effects and possible scope frequency response? Can you use, for
example, 1 KHz?
The 1nF and 1500 ohm input network give a corner frequency of about 100 KHz. Can
you tack a 1 uF across the 1 nF and drive it with 1 KHz? This sets the corner to
100 Hz, so there should be a good signal at the base.
The output signal should now be undistorted. Then by raising the test frequency,
you can see the frequency where the distortion begins. This should tell us if
there is some capacitor inadvertently connected to the output and the approximate
Just to check, have you tried using another transistor? I can't think of a
failure mode that would give these symptoms, but I suppose it's possible.
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