[sdiy] Patchell's Boggling 13700 Expo Converter Thing???

[Magnus Danielson]

From: Ian Fritz <ijfritz at earthlink.net>
Subject: Re: [sdiy] Patchell's Boggling 13700 Expo Converter Thing???
Date: Sat, 21 Jun 2003 21:29:58 -0600

> Cynthia, et al. --
> 
> It's a boggler, all right.  I spent a big chunk of time a few weeks ago 
> analyzing it in great detail, including working out all the possible 
> temperature drift sources that I could think of.  The most amazing part 
> about it is the tandem OTA part, which casus the usual OTA input 
> nonlinearities to cancel to a *very* high degree of accuracy.  These 
> nonlinearities were the biggest drawback of other designs that people here 
> have worked on.

Ehum! May I point out that non-linearities on _linearized_ OTA inputs is
greater than they possibly could be since the common approximative curcuit
leaves much to desire. The linearized OTA must be driven by a diffrential
current source with a stable common mode current in order to operate correctly.
The normal fashion of doing things is by tossing in a few resistors, which is
not the best way to acheive the diffrential current source.

Jim's design does take another interesting approach, but it may not be the only
approach to get there. I think again habbit works over analysis mode.

Cheers,
Magnus