[sdiy] CMOS chip questions

[Florian Teply]

Am Sunday 19 July 2009 20:13:16 schrieb Scott Gravenhorst:
> "David G. Dixon" <dixon at interchange.ubc.ca> wrote:
> >> Why Would one need any TTL interfacing? Just tell me what's missing on
> >> CMOS replacements for TTL chips. It's probably 7404?
> >
> >Samppa's definitely got a point.  Mixing TTL and CMOS is kinda like
> > putting '74 Chevy Impala parts in your Acura TL.
>
> There are times when one needs more drive current than CMOS provides.  One
> way to deal with that is parallel drivers, but another is bipolar TTL.  I
> guess it depends on exactly how much current one needs.  I think, however,
> that for most synthesizer designs, such currents are more rare than common.
>  Stepper motors?  But who puts a stepper motor in a synth?
>
And even if one would need more juice to drive stuff, one would have to 
consider that TTL isn't much better than CMOS at sourcing currents due to the 
typical design of a TTL output stage. Sinking the current (maybe with some 
Open-Collector output stage) is another chapter though. And for another power 
stage afterwrds, it doesn't really matter whether the chip itself is TTL or 
CMOS. Unless there are specific needs (certain types of gates not available 
in some sort of logic family), i would go for straight CMOS anyways due to 
their lower power. Not that it would matter on a single board, but in a large 
modular synth, this would make for the odd amp or two in power supply...

Just my 2 cents,
Florian