interesting VCO was RE: [sdiy] Imperfect VCO

[ASSI]

On Monday 04 August 2003 17:32, Scott Gravenhorst wrote:
> I was hoping that the
> capacitor charge switches could be implemented using a 4069 or 4007,
> but it seemed to me that the with a current source at the top of
> that, we would have problems with the static protective diodes.  Am I
> close with that? 

You would need to make sure that the input is never more than about 
0.4V above/below the supply terminals. The parts needed to ensure this 
probably make it not worthwhile compared with other solutions (unless 
of course you are going to use all inverters in the package and find 
some external components that match in density).

> If so, I suppose the cap switches could be built
> with discrete MOS transistors or what about a 4066 to do that?

A discrete MOS inverter is certainly possible, you don't even use it in 
linear region, so no worries about matching. Zetex makes some dual 
"logic" MOS in SMD packages that have low enough threshold voltages and 
not too high gate capacitance. These should work well.

The 4066 uses transfer gates, but again you feed it with current and so 
the nonlinearities of the Ron are taken care of already. There is not 
much point in trying to make a 4066 out of discrete components unless 
you want to make a "power" version, as it's not easy to match the n- 
and p-MOS of the switch.

> Wouldn't that solve the diode problem (assuming there is one)?  If it
> could be done with 4069, that seems to be simpler perhaps.

I'm not sure if a 4069 is available as a single gate (tinyLogic or some 
such), then it would be easier to perhaps work around the protection 
diodes.


Achim.
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