Re: Current source design (tricky?)

From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 03/12/05 

Date: Sat, 12 Mar 2005 13:58:39 -0800

"John Larkin" <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX>
 wrote in message news:34m631dbl759hd9g8vmhd119ku9mbap6je@4ax.com...
> On Sat, 12 Mar 2005 12:37:53 -0800, "Larry Brasfield"
> <donotspam_larry_brasfield@hotmail.com> wrote:
>
>>The "capacitive load
>>accomadation" feature described for the LM8261
>>is strictly limited to large signal conditions.
>
> But it's not. If you reflect the "Miller" capacitance to an equivalent
> capacitance at the output node, adding external C simply makes this
> net C look bigger. They have merely moved the dominant pole to the
> output pin. It's not much more profound than putting two capacitors in
> parallel. If the c-load fix only worked large-signal, it would
> oscillate at low level when c-loaded, and it doesn't.

Ok, I can see that now, having searched for "miller" in
the data***. My comment arose from examination
of the language under the the heading "DRIVING
CAPACITIVE LOADS" where Miller effect is not
mentioned and a mode change with large error input
is the described effect.

The effect is clearly different from the dominant pole
shifting that occurs in devices such as the LM1812,
which is what I initially was describing.

> These *are* great parts for driving ADC references and mosfet gates.
>
> Too bad the input offset shifts with common-mode voltage, as most r-r
> front-ends still do. I think there's a subtlety there as regards not
> making a schmitt trigger out of the whole thing.

It would be quite a trick to avoid the offset
shift when the effective input pair changes.

>>how one might confuse it for Miller effect is a
>>puzzle. Care to explain that, Fred?
>
> The National data*** calls it Miller Effect, but that's sort of an
> ancient blanket term for any sort of capacitive feedback.

I think NS is not abusing the term there. It
is a natural extension to substitute "simple
inverting gain stage" in place of the tube or
transistor that appears in early descriptions of
the Miller effect.

> Fred's not at all nice, but he's usually right.

Perhaps I have not had the stomach or time to
peruse enough of his posts, but my sampling
indicates otherwise.

I must say that it is a lot easier to understand
somebody's points when they are not buried
in a stream of invective. I appreciate your
reasoned approach to this. 

-- 
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.