[sdiy] Buchla 258/Plan B M15 expo converter

[JH.]

> Hopefully my reasoning in my reply to Ian shows that (annoyingly) it is 
> not
> as simple as an emitter-follower (hence the problems I'm having analysing
> it), but amplifying in some way - probably yes.

Just draw the circuit without Q12 (or look at a VCS3 VCO, or my Living 
VCOs), and it's clear what Q12 does in th eBuchla circuit.
Amplifying, yes: current!

JH.



>
>> The EMS VCOs use the same method, but without
>> Q12. With EMS, Q1 has to take all the excess current from R20
>> directly (what is left over from the reference current plus
>> the output current of the expo transitor pair). With Buchla,
>> only a fraction of this excess current (ratio set by 33k and
>> 6k8) flows Q1 and the zener diode, keeping the reference
>> cooler (or allowing higher output current for a given current
>> thru the reference).
>
> If it were an op amp instead, I (more or less) don't have difficulty
> believing that the op amp can 'mop up' the 'excess current'. For this
> transistor case, it is quantifying all the gains etc. around the 'servo
> loop' that I want to see, in order to convince myself that it *is* also
> capable of doing the same thing!
>
> However, the fog may be lifting with respect to these standard feedback
> configurations - I'm going to work through the simple emitter-follower 
> case
> to see what I can learn, and so see if it is extendable in some way to
> putting such a follower in the feedback loop, which looks to be near 
> enough
> what these setups are doing.
>
> Tim
> __________________________________________________________
> Tim Stinchcombe
>
> Cheltenham, Glos, UK
> email: tim102 at tstinchcombe.freeserve.co.uk
> www.timstinchcombe.co.uk
>
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