Re: Op amp darlington current source oscillations

From: Justin Dobbs (jrd_at_jrd.com)
Date: 09/14/04 

Date: 14 Sep 2004 08:35:47 -0700

You conveniently cut out a key part of the original post:

> > This circuit initially had just the first op-amp stage driving the
> > darlington pair though the 10-ohm resistor, with no lowpass filter,
> > and the behavior was the same.

To clarify, this is what was oscillating:

> > Rload=8ohm Lload
> > 12V |--------------/\/\/\------88888----------------------.
> > |
> > 5V |
> > --- .1uF |
> > |---| |---. |
> > | /// |------.
> > | | |
> > 10k |\| | |
> > ---/\/\/\---------|+\ 10 or 0 |C |
> > | | >-----------------/\/\/\-------------| _|_/
> > | . --|-/ |E //\
> > -- | |/| | /__\
> > .1uF -- | | | |
> > | | /// | |
> > | | | |
> > /// | | |
> > | | |
> > | | |
> > |___________________________________________+______|
> > |
> > |
> > \
> > / 0.2 ohm
> > \ 5W
> > /
> > |
> > |
> > ///
>
>
> Take the first stage out completly and connect the feedback to the -input of
> the remaining amp. The 10k/10u will give a good stable DC voltage from the
> PWM control. Learn about opamps and try to understand how they work apart
> from cpying bits and putting them together. This is *not* software.

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