Re: Current source design (tricky?)

From: Terry Given (my_name_at_ieee.org)
Date: 03/11/05 

Date: Sat, 12 Mar 2005 12:09:08 +1300

Larry Brasfield wrote:
> "Genome" <ilike_spam@yahoo.co.uk> wrote in message
> news:EcmYd.1005$tb7.730@newsfe6-gui.ntli.net...
>
>>"Winfield Hill" <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote in
>>message news:d0pph4024j2@drn.newsguy.com...
>
> ...
>
>>> There are a few subtleties in such a circuit, such as isolating the
>>> opamp from the high gate capacitance of the FET,
>
> ...
>
>>If your op-amp is loaded with a capacitor then its internal thinging
>>resistance between the emitters of the output emitter followers forms an
>>extra pole in the circuit with the mosfets gate capacitance and that adds to
>>the transfer function and upsets things. The resistance is internal to the
>>loop that you add around the op-amp.
>
>
> I don't want to burst any balloons here but ...
>
> 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 application circuit was a HPF driving a
diode frequency discriminator (aka leaky charge-pump). Both the feedback
(120pF) and DFD (120pF) caps conspired to completely *** up the HPF -
to the point of uncontrolled oscillation. Re-jigging the HPF and DFD to
use 56pF caps helped, but the in-circuit GBW was still too low (as
measured by filter stability). I could have re-designed the HPF to
tolerate the actual GBW, but had no idea what the production spread was,
so went and found an opamp without this "feature".

Said "feature" btw is designed specifically to drive capacitive loads up
to 500pF! which is why I picked the rat *** POS ITFP! Doh!

>
> Adding a load isolation resistor to the output
> tends to defeat that feature, so I would not
> add it unless using an op-amp without that
> bandwidth reduction feature.
>

Cheers
Terry