Re: [sdiy] Daffy CMOS ASCIImatic

[jhaible at debitel.net]

Just a thought: 
FETs as linear VC resistors need a rather small signal level.
OTOH linear amplifiers built from 4000 series CMOS chips
are not exactly low-noise, so they benefit from rather high signal
levels. 
Both are good ideas, but I would not necessarily use them *together*.

I have often thought how nice it would be to have a "low noise" version
of a 4007 or a 4069. Of course it would be possible to do similar things
with discreete transistors, but that's expensive, and the elegance of the
creative use of a low cost 4007 would be lost ...
(I think there used to be a - rather expensive - CA3xxx part with a CD4007
configuration, but I never checked this.)

I have not thought this thru entirely, mind you. So I cannot quantify
"noisy" at this point. Maybe part of it just comes from the high resistor
values that are typical for linear 4000-series applications. (If you're
using small feedback resistors, they are loading the amp outputs, which are
just two current sources in open-loop case, more or less.)

JH.



At 19:07 28.04.02, Scott Gravenhorst wrote:
>Or a dual supply, +/- 7.5 Volts  and then the load goes to 
>ground ?

Right, something like that.

>I thought about this, couldn't figure why...  I assume you
>mean to use the 2nd linear inverter like a cv preamp to this?

The output of the "switch" would be run directly into the 
inverting input of that invertor.

>I don't get this.  Which input and why?  CV in?  I've seen 
>this in single transistor units (drain to gate) but it's 
>coupled with a resistor.  Is this a linearity improvement?

If one uses such a configuration, the output of the two MOSFETs is 
held at a constant potential. This should improve linearity.

>Also, I looked at this again, and I think that if only one inverter
>is used, this can be made out of one 4007.  Powering it will have no
>effect on protection diodes turning on.  Only an analog signal out
>side of the rail-rail boundarys would cause problems.  Also I think
>the other pair of transistors could make another VC resistor that
>reponds reverse of the first.  VC pot?

That would be interesting.

Cheers,
 René