[sdiy] Quick query about 2164 VCAs

Neil Johnson neil.johnson71 at gmail.com
Mon Mar 18 23:52:40 CET 2019

Hi Matthias,
> BUT it assumes identical units! Offset voltages, offset currents and other imperfections of the real world would break the spell, possibly forcing one VCA to react much much more and gulp up much more of the input current than the other VCA, leaving us with a circuit more similar to a single VCA without higher current capability or lower noise.

Indeed, but the mental model of the front end of the 2164 as an
opamp's virtual earth is just that - a convenient mental model.  It is
good enough for a lot of reasoning when considering just a single VCA,
but when you connect several of them together that model no longer

> So, in the real world, even with VCAs on the same chip: How close and well-matched can they possibly be, in the worst case? Is it reasonable to believe that all the error differences are orders of magnitude smaller than the already tiny (but crucial for the function) non-zero voltages at the virtual ground inputs? What do you op-amp experts say, can you connect op-amps fully in parallel without worries? :-)

For op-amps, see the work of Doug Self.  For the 2164, yes under
certain circumstances!


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