At 10:18 AM 1/5/01 -0500, you wrote:

>It also has to be a CMOS inverter, which is really an amplifier with a gain
>somewhat less then -1.  TTL and other logic families won't work.  And of
>course don't use a schmidt trigger.

It has to be an *unbuffered* (UB) CMOS inverter to get good results, though as
some have said you can sometimes use LSTTL inverters etc. for oscillators.

The problem with using a buffered version is that there are internal nodes
that you cannot get at in order to compensate the amplifier to get some
phase margin. It can thus oscillate internally (which is a BAD thing).

Actually the gain near the transition (where you bias it) is much more
than 1, you can reduce it to some controlled value by using a couple
of resistors.
                       R2
                x----/\/\/\/\---x
   C1     R1    |               |
0--||--/\/\/\---x-----|>o-------x----> out


Gain ~= -R2/R2 , for reasonably low values of R2/R1
Input impedance ~= R1 for Xc << R1
Vout (dc) ~= Vdd/2 (but quite variable, see the data sheets)

I'd suggest only doing this with 40xx series CMOS, the Iq of the
parts such as 74HC and so on may be quite high.

Best regards,

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