Posted by
wb2srf on
Sep 09, 2007; 7:39pm
URL: http://elecraft.85.s1.nabble.com/KX1-speaker-output-with-superimposed-oscillation-tp452289p452293.html
Hi Paco,
Great detective work, glad to read that you tamed the beast. Hi Hi
I am in the process of trying to model the LM386 circuits using LT spice
( program free from Linear Technologies web site SWCAD III) to try to
understand the feedback mechanism that results in these amplifier's
becoming unstable and turning into an oscillator.
Many times I have had success through trial and error getting circuits to
work properly but with the help of some engineer friends, I have been
encouraged to try circuit analysis. It is strange that placing a
capacitor at the output to ground solved your problem because it can
cause a phase shift to result acting with the output resistance internal
to the IC. Perhaps the phase shift placed the gain-phase margin outside
of the area in a Bode plot to stabilize your amplifier
These amplifiers have a problem. Before they run out of gain at high
frequencies, if there is 360 degree's of phase shift from input to output
of the amplifier it turns into an oscillator.
73,
Bob Johansen WB2SRF
On Sun, 09 Sep 2007 16:28:43 +0000 "Francisco Barros"
<
[hidden email]> writes:
> Hi Bob,
>
> Finally, I got it !.
> It is related with "the RC network connected between the output pin
> 5 and
> ground, required to keep the amplifier stable at high frequencies at
>
> different loads", as you mention on your previous post.
> I checked the RC network and I even installed a new one: same
> behavior.
> If I removed this RC network completely, the oscillation now started
> with
> lower volumes !.
>
> Using the technique "try and error", the problem was solved with a
> capacitor
> of 68 nF in parallel with this RC network, that is, just from pin 5
> to
> ground. No more high frequency oscillation with different loads !.
>
> By the way, the DC voltage at pin 5 was half the supply: 3 V. I even
>
> installed a short leads 10 uF tantalum across the supply of U4,
> without
> positive results.
>
> I don't know the reason of this behaviour, maybe, as you say on your
> post,
> is due to the high gain of this stage (46 dB). Who knows... .
>
> Many thanks Bob and have a good day.
>
> 73,
> Paco
>
> _________________________________________________________________
> Descarga gratis la Barra de Herramientas de MSN
>
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>
>
>
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