vcf5t update

urosuros urosuros at bits.net
Mon Mar 6 03:39:19 CET 2000


> > Are you shure this is conected right ?
>
> It seems correctly hooked between +Vcc and -Vcc, yes.
hi
thank you Magnus I realy got mixed up thinking how it could work with single
power suply
> > Also , in old MS50 filter schematic from EFM we have 3046 array in diode
> > conection. Are you shure Tom that 1N914 will work here ?
>
> Notice that the 3046 transistors are wired up to act as diodes, only the
> base-emitter NP-junction is being operational. The sourcing diodes has
been
> replaced by the 100k resistors.
I know that but I was considering diode maching ( btw are trans in3046
mached )
since diode transconductace will depend on temperature ( as in expo
convertor )
also should diodes Vt should be mached ?



>
> > And two question not regarding this filter .
> > First if transistors in ladder filter are not matched that would decrese
CV
> > rejection but what else will be affected ?
>
> Yes, and it would also decrease the common mode rejection of the input
stage,
> to what ever degree that is usefull.
>
> > It seems to me that poles would spread a little ( or a lot ) and that
would
> > decrese resonance and overall filter performanca , right ?
>
> Yes, the unmodulated poles would not lie on the same place, so the initial
> part they would be dragged together and only when the feedback would get
high
> enougth they would spread out as we are used to. I don't think the really
> interesting thing is happening in the linear frequency plane (that is pole
> positions) but rather in what happends in the non-linear range with
operation
> points of the individual transistors.

wait a second if we interpretate ladder as four integrators conected
in series transistor missmach would be equal as integrators having different
RC and thus their poles would be spread in freq plane ( i was not thinkig
of s-plane spreading which happens with feedback increase )
so we would not have resonance overthrow ( or peak if you
prefer that term ) with feed back increase so moog ladder would sound
like nothing
bottom line : if I want to hand mach transistors for ladder how
well mached should transistors be ( and btw do i need to mach
anithing else except beta ) and do the perfectly mached transistors
drasticly improve performance


>
> > Could this be neglected by simply increasing feedback around ladder ?
>
> Nope, the feedback will raise the Q-value of two of the poles and move
them
> closer and closer to the jw-axis where as the other two are moving away
from
> the jw-axis. The poles are sitting on the 45 degree positions of an
expanding
> ring, the ring expands with the increased feedback.
>
> > And second : dif amp with active load has best performance in CV
rejecting
> > in discrete VCF and VCA configurations ) but I need three mached pairs
 for
> > mirror in emiter , for active load , and for difpair ) Is there any
simpler
> > configuration ( simpler = less parts and less matching ) with good Cv
> > rejection ? The simplest would be one common colector transistor with
> > controled curent source in colector but it would have terrible Cv
rejection
> > and could only work in VCA ( has anyone seen VCF built around this ) ?
>
> Have you considered having another diff-pair to follow up? Let the inputs
of
> the second diff pair be at the collectors of the first diff pair. Just a
> thougth.
hm , what should i need for second diff pair ? ( all IC ota's use just one
transistor pair )
maybe dinamic range increse ? and that would mean more parts ( larger pcb )
and more maching . Im interested in findeing out is there simpler
transconductanse core than
dif pair with current sink in emitor

> There are books describing such things, Gray and Meyer springs to mind.
>
> Cheer,
> Magnus
thank you very much
uros





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