[sdiy] Buchla 258/Plan B M15 expo converter

[Ian Fritz]

Hi Tim --

Here's my take on that circuit.

The 6V Zener D1 and the PNP Q1 maintain a constant voltage of 6.6 V across 
the 330k R21, thus producing a constant standing current of 20 uA into the 
collector of the left side transistor of IC3.

As the base voltage of the left side transistor is varied by the input CV, 
the tied emitters float to whatever voltage is necessary to maintain the 20 
uA standing current, ie, the left side transistor is basically an emitter 
follower with a constant collector current.

Since the base of the right side transistor of IC3 is grounded, there is a 
delta Vbe across the pair, leading to a current on the right side 
transistor that is exponentially related to the input CV, in the familiar 
manner.

I'm not sure about the purpose of Q12.   It keeps the collector voltage of 
Q1 fixed near 0 V.

   Ian





At 03:24 PM 8/17/2010, Tim Stinchcombe wrote:
>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
>
>
>
>
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