[sdiy] Buchla 258/Plan B M15 expo converter

[Tim Stinchcombe]

Hi Ian,

> 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.

Yes, all is well and good to this point. A little further simple reasoning
reveals a bit more: since the left transistor is merely following the input,
the total current through R20 is simply linearly dependent on the input
voltage, but nearly constant (the emitters only move by 100s of mV or so);
since the output through the right transistor is exponential (like e^(-x)
wrt input x), subtracting this from the nearly constant R20 current, and
subtracting the standing current (through R21), shows that the current
through Q12 is along the lines of '1-e^(-x)', which actually looks very
log-like. Thus Q12 is certainly acting in the large-signal sense (as the
right transistor is too of course) - hence its b-e voltage is not constant
at all, so Q12 is not acting anything like an emitter-follower!

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

Near ground, yes; but going by the above, the collector voltage will not be
that fixed! (Simple SPICE simulation bears this all out.)

Tim
__________________________________________________________
Tim Stinchcombe 

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