[sdiy] CMOS for linear circuits.

[harrybissell]

The theremin is a VERY special case.... any interaction bewteen the
oscillators will lock them and prevent beating, which will kill the
theremin dead !!!

Usually I use gates in the same package ~if~ I don't care about
interaction. That goes double for opamps... try putting a preamp
in 1/2 an IC and a comparator in the other half :^P  (wrongo!)

Power dissipation was mentioned.

CMOS 'linear' operation is really a bad idea all in all. If you only need
~one~ amplifier and you have a spare gate... maybe.

I was really really unhappy with using a 4049 in linear mode in a theremin
'octave divider'.   The self-biasing keeps it right at the most sensitive
region
and then the slightest noise (like RF :^) makes it go nuts.  Some folks
would
have liked the "bastard sync'd oscillator from HELL" effect that I got (I
was
tempted to make it a feature) but cooler heads prevailed, and I used a
comparator
instead.

I don't call most simple RC delays, schmitt trigger oscillators etc...
"linear"
CMOS. I think those are legitimate uses. Its when you use feedback that its
really 'linear'.   A 741 will kick CMOS @ss in "linear"

H^) harry

ryan williams wrote:

> hi all,
>
> since I'm currently building some cmos filters, vcos, and theremin
> control circuits I have been wondering about using multiple gates on the
> same part.
>
> the glasgow theremin
> http://home.att.net/~theremin1/Glasgow/glasgow.html
> uses all CMOS to generate a control voltage from hand-antenna distance
> (just like any other theremin, but with tachometer). They have used a
> different IC for each gate that sees a frequency that is different from
> the others. There is 10 ICs total just to make up the antenna->CV
> sections. there are enough available gates to do this with only 5 ICs.
>
> Whats the deal with this. I expect there might be some coupling between
> gates on the same chip? I have not experienced something like this but
> only read people mentioning that kind of problem. So when is it ok to
> put them on the same chip, and when should they be seperate? maybe it's
> a complicated question. if so, what books to read?
>
> -ryan