[sdiy] Buchla 258/Plan B M15 expo converter

[Tim Stinchcombe]

> Q12 is just an emitter follower / current amplifier inside 
> the servo loop.

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.

> 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