[sdiy] Question about my high freq compensation

[Ray Wilson]

Fantastic feedback! Thanks for your time and effort. I will be back at the
breadboard with all of these suggestions and I think my oscillators will
track far better as a result. I'm just about to endeavor onto a whole new
modular. I built an actually nice looking wooden case, have experimented
with a good labeling scheme and now I will implement these great
suggestions.

Thanks and Cheers

Ray


----- Original Message ----- 
From: "René Schmitz" <uzs159 at uni-bonn.de>
To: "Ian Fritz" <ijfritz at earthlink.net>
Cc: "Ray Wilson" <raywilson at comcast.net>; "Synth-Diy"
<synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, December 10, 2003 12:30 AM
Subject: Re: [sdiy] Question about my high freq compensation


> Hi Ian and all,
>
> > Second, everyone should be reminded of the clever improvement on this
> > compensation circuit developed by Rene.  Rene's circuit has a 10k trim
> > pot connected to the servo output, followed by a diode to ground.  The
> > voltage from the trimmer's slider is fed back to the input base through
> > an appropriate resistor (depending on the amount of parasitic
> > resistance).  This structure more nearly mimics the output side of the
> > converter.
>
> Well it appears I never really published that circuit. I will post it
> later. (IIRC, we were discussing this in private back then, so most
> people won't remember it. :-D )
>
> > Finally, I would agree with Oren that many of the advantages of using
> > the expensive, high- performance transistor pair are obviated by the
> > relatively slow switching circuit and also by op-amp offsets and
> > leakage.  If fact, my most successful designs have used the exact
> > opposite approach of yours, namely medium performance converter
> > transistors (CA3083 array) and high performance op amps and switches,
> > along with separate Rbe and reset-time compensation.  At some point you
> > might want to try using a less expensive pair and looking at whether the
> > performance is really much worse.
>
> 150% Agreed.
>
> For example rearranging the TC to a divider scheme (instead of using it
> as the feedback resistor of the summing amp) could yield a bigger
> improvement on stability than changing the expo transistors. And also
> would allow standard 100k input summing resistors.
> To me precision design is mostly choosing good circuit topologies.
> Simply because "ideal" parts almost never exist.
>
> Cheers,
>   René
>
>