When trying to bias a logic gate in the mid-region for analog
operation, you generally have to use an "unbuffered" gate. Most logic
gates are actually several stages in series to help improve the
device's gain (a single stage would generally only have a gain of
around 50 which would make the transition between low and high output
states be more like a ramp than a sharp jump). Buffered gates are
often not stable when biased mid-way.

Some logic families may be unbuffered by default. Others, like the
74HC series, are normally buffered and you have to get the UB version
(like 74HC04UB) for a true single-stage inverter.

Sean


On Fri, Feb 20, 2009 at 11:44 AM, Gerhard Fiedler
<lists@connectionbrazil.com> wrote:
> As so often, Wikipedia is useful :)
>
> <http://en.wikipedia.org/wiki/Biasing_(disambiguation)> leads you to
> <http://en.wikipedia.org/wiki/Biasing_(electronics)>. The first phrase
> says:
>
> "Biasing in electronics is the method of establishing predetermined
> voltages and/or currents at various points of a circuit to set an
> appropriate operating point."
>
> That's basically it. What Larry wrote about is that a logic gate usually
> has two main operation points (lo and hi). The area below lo and above
> hi is generally a no-no (you're likely to destroy the device). However,
> between lo and hi the device can be used, even though it's generally out
> of spec. In that area, the digital part works as an analog part.
>
> In short: When used as an oscillator, you have a (small) signal that
> oscillates. To this small signal is added a "bias voltage", to keep the
> sum in area between hi and lo.
>
> Gerhard
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist