Re: Current source design (tricky?)

From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 03/11/05 

Date: Fri, 11 Mar 2005 15:56:36 -0800

"Terry Given" <my_name@ieee.org> wrote in message
 news:qypYd.8841$1S4.942601@news.xtra.co.nz...
> Larry Brasfield wrote:
...
>> A number of op-amps on the market today are
>> very tolerant of capacitive loading because they
>> have a feature whereby that loading causes the
>> gain-bandwidth of the part to drop, almost in
>> proportion to the loading, such that the extra
>> pole remains far enough above the unity gain
>> crossover frequency that stability is preserved.
>> The LM8261 suggested by Mr. Hill is a good
>> example of this class.
>
> I have been bitten quite badly by a similar "feature" in
> the LM6134 (its a slew-rate modification).

The feature I mentioned above works by causing the
effective value of an internal capacitance to increase.
So it changes both the linear small-signal response
(less GBW) and the slew limiting (slower).

The adaptive slewing feature that National (sort
of) describes in the LM6134 data*** is not the
same thing at all. It operates by increasing the
amount of current available for slewing under
certain large signal input conditions.

As for the problem you had with it, I would not
deem it a biting feature so much as a reason to
not use it without understanding it better. I will
say, however, that mode changing circuitry for
the alleged benefit of large signal conditions is
something that usually gives me the willys. 

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