[sdiy] Question about my high freq compensation

[René Schmitz]

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