CMOS linear warnings...

[Harry Bissell]

Hi y'all

1) the CMOS as linear stays biased in the (duh...) linear region, so it runs
at a much higher current.

2) at the bias point, the units are very prone to noise pickup and oscillation. I
tried to
"cheat" and use a 4049 as a linear amplifier in an Etherwave (rf neighborhood) and
it
was NOT GOOD...  I won't do that again. Wasted a day trying to stabilize, then
used a LM311 which worked right away.

3) IF you want sh!tty operation, as in guitar fuzzbox... go ahead.

4) The Wasp Filter uses this config... perhaps it works better as the feedback
makes
those linear stages integrators... which would tend to suck up any self generated
noise...

5) only use unbuffered chips. using the buffered type will result (likely) in a
latch-up and
burn.

6) use only in desperation.... Thats why they make "op-amps" anyway...

H^)   harry    <don't need flamesuit tonight...>

Magnus Danielson wrote:

> From: bader <trip at blackstar.myip.org>
> Subject: Re: Hex VC mixer
> Date: Mon, 24 Jul 2000 13:22:16 -0500 (EST)
>
> >
> >
> > While were on the subject of digital logic for analog functions, This
> > reminds me of something i saw with a 4011 that made it function
> > as a quad op-amp. Connect one of the inputs to vcc, and the extra input
> > acts like the negative input on an op-amp. Add your resistors and
> > feedback loop, there you go.
> >
> > I haven't tried this myself, Same idea, maybe?
>
> Not quite, but it is up along a similar path thougth. Digital logic is really
> just digital since we keep pounding it into your poor heads, heck, there is
> no digitial chips around just yeat, it's all very analog. It is however
> convenient to say its behaves like digital logic. Anyway, this is how it works:
>
> Take any CMOS inverter and sweep the input from rail to rail and you will see
> a curve like this:
>
> ----
>     |
>     |
>     |
>     |
>      ----
>
> Now, in the steep slope in the middle the inverter is acting like a linear
> amplifier with a negative gain. The trick to use this gain is to bias the
> input so that the signal is operating in this linear section. You can do this
> to a 4049 using a resistor between input and output and then a resistor to
> either rail (no, it is not the VCA operation). Now, I haven't thougth of using
> an 4011 as a quad op-amp, but looking carefully at it I see that you can use
> the same biasing trick to get it going.
>
> However, both inputs would be negative ;)
>
> > On a sidenote, maybe you could use the input connected to vcc to toggle it
> > between inverting an non inverting. Once again, i haven't tried this.
>
> Hmm... I do not think so....
>
> Now I just await for someone to propose how to use an Z80 or i80386 as a
> compressor, just by setting it up correctly in analog operation mode ;)
>
> Cheers,
> Magnus