moog hp

[Gene Zumchak]

List,

    I am looking at tomg's gif of the moog_hp that was attached to his email of
March 28.  (I'm attaching to this reply for your convenience).  It is very
curious.  There is no place for the current of one transistor to go than through
the other transistor.  This has to happen independent of any matching between the
NPN and PNP pair.  The control voltage defining the current through the NPN or PNP
groups whose bases are all tied together is feed through a common resistor.  While
the resistor would allow an NPN to adjust to the PNP a single resistor serves all
NPNs at the same time.  How can they all adjust independently.  It is not at all
clear how the transistor collector current (and hence its effective resistance) is
defined.  Typically, for controlling transistor transconductance you either nail
down the base and diddle the emittor or vice versa.  In this circuit, it is
unclear at what voltage any of the emmiters are biased to.  It is obvious,
however, that within an NPN/PNP pair, it is the emitter voltage which must adjust
itself in order to cause both the NPN and PNP to have the same current.

    In short, from the circuit, it is not clear what transistors have to be
matched, and what will happen if they are not matched.  For me, anyway, this is a
most interesting and difficult circuit to analyse.  I will appreciate help from
anyone (Juergen, Jorgen, Michael, Harry, et alia?) who can enlighten me.

    Next, the output is taken off the last filter section single-endedly, and
there is no provision for control voltage rejection.  Both the audio signal and
control voltage component appear to be amplified by a factor of ten.  Thanks in
advance to anyone who can shed light.

Gene Z.



WeAreAs1 at aol.com wrote:

> Hello Juergen,
>
> In a message dated 3/29/99 3:16:22 AM, you wrote:
>
> <<Seriously, I was wondering why Moog used transistors
> instead of diodes here. Matching diodes should have
> been easier than matching npn / pnp pairs.>>
>
> Can you offer any suggestions for a transistor matching procedure that will
> match NPN and PNP pairs?  Will the Moog matching technique work?  (as
> published in the Minimoog service manual)
>
> How about matching diodes?  Do you have a special technique for this?  Anybody
> else have any tips?
>
> Michael Bacich
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