[sdiy] Buchla 258/Plan B M15 expo converter

[Tim Stinchcombe]

Hi list,
	I have been attempting to analyse the Buchla 258 expo converter, as
adopted by the Plan B M15, but have virtually run to a standstill. It is D1,
R21, Q1, IC3, Q12, R20 & the 33k here:

<http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2580_1_20
0.jpg>

The basic operation is pretty straightforward, assuming that Q1 & Q12
replace the op amp often seen at that part of the circuit, as for example
U1B, R15, Q1, R16 here:

<http://rubidium.dyndns.org/~magnus/synths/friends/stopp/asm1vco.new.pdf>

However, trying to enumerate the gain etc. of Q1 & Q12 (258 circuit) is
where I'm struggling - after many SPICE simulations and several sheets of
algebra it is clear my approach (mostly using the simplified Ebers-Moll
model for the transistors) isn't going anywhere.

So having just spent several hours reading about the four basic feedback
configurations (shunt-series, series-series etc.) it looks like _that_ might
be a good way to go, as the methods allow for breaking the loop/circuit down
into constituent parts so as to 'isolate' different parts from each other,
but still allow for the effects of the bits discarded (if you get my
drift...).

But now here is my problem: finding op amp circuits to be generally easier
to grasp than transistor ones, I'd like to couch the standard *op amp-based*
expo converter (as the ASM VCO above) in terms of one of these feedback
configurations, but I'm struggling with how to deal with the input
transistor (Q1 in ASM circuit), and 'lump it in' with the main amplifier (if
indeed that is the thing to do). (I'm assuming the output transistor Q2 just
looks like a load to the op amp, so is less of a problem.)

So, does anyone know:

1. if the standard expo converter *can* be put into one of the four basic
configs, and if so, how, and which one? (that transistor looks awfully like
it might be a *3*-port, rather than a 2-port, in the feedback loop, and
hence invalidate the method...?)

2. if there is already a decent analysis of the 258 expo circuit lurking
around somewhere?

I've found little in the archives, and Googling 'feedback configuration expo
(or 'antilog') amplifier (or 'converter')' etc. yields so many hits as to be
useless (and so you start doing things like '-syllabus', '-syllabi' to
exclude most of what is spat out, till you apparently get to the limit of
the number of parameters Google will allow so it simply ignores them...
great :-( )

TIA,
	Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk