[sdiy] Buffers on the LM13x00

[René Schmitz]

Hi Jürgen, Aaron and all,

> In most applications, a tight (at DC) feedback loop takes care of this.
> An OTA + cap + buffer is an opamp, with I_abc you just change its
> transit frequency and slew rate. Yes, the DC too, but to a much smaller
> degree.

But as Iabc decreases, so does the loop gain. Hence a Iabc dependant 
output offset would exist there ((*) at least in theory). In the extreme 
case, the OTA cuts off completely and the darlington buffer sits at 
Uout-1.4V. Then again the output voltage isn't really defined anymore, 
unless you have some additional path to GND, this may change the 
behaviour somewhat.

(*) I must admit that I've never really used the LM13x00 much at all, 
somehow I'm a 3080 guy.

As to why some use opamps as buffers: A jFET input OP does have a much 
higher input impedance than the darlington buffer, which gives better 
performance at the low end of the Iabc range. The only thing to watch 
for with the OP buffers is phase reversal.
Sometimes its also just a "backport" from 3080 schematics, where the 
opamps were originaly used. Most of the time designers don't reason 
about this too much, if the thing works ok.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159