SEM VCO buffer

[harry bissell]

OK so I was wrong about temperature performance of CMOS schmitt, its really
quite good over a very wide range. Not guaranteed at any one value, but very
stable at wherever it is.

I have a copy of Nat Semi App Note 140 which gives the schematic and expanation
of how the schmitt trigger works. Real interesting stuff. I'll scan part of it
later and post.

:^) Harry

Martin Czech wrote:

> :::     >Another idea is to use a CMOS logic Schmitt trigger IC.  One of the
> :::faster
> :::     >logic familys.
> :::
> :::The ARP Odyssey uses a CMOS gate to detect the threshold (no schmitt
> :::trigger, though).
> :::
> :::How are CMOS schmitt trigger made (internally), btw ?
> :::I know how to configure one from two inverters and two resistors, but I
> :::have no idea how the genuine CMOS schmitt triggers are made.
> :::
>
> Juergen,
>
> the two inverters/two resistors type is the typical kind of circuit we
> find in books only. In reality (at least in ICs) something different is
> used (no good resistors in CMOS logic process).  It is a design that is
> copied now for ages, nothing sophisticated, just 2x3 MOSFETS and maybe
> another 2 for a buffer.
>
> If you have an old CMOS CD40xx databook I'm sure that you'll find the
> schematic there. The feedback is done via currents. This simple circuit
> is very dependend on Vdd (no problem) but no much on temperature (which
> is good).
>
> m.c.