1000uF on a low output impedance emiter circuit ?
The output capacitor should be low, 10uF up to 100uF..
You must have resistors on both OA inputs.

Vasile

On 9/11/06, Sean Breheny <shb7@cornell.edu> wrote:
> Hi all,
>
> As you all know, linear regulators essentially consisting of an
> op-amp, voltage reference, and pass transistor are very common. One
> drawback is that they cannot sink current, only source it. The first
> time I tried to design such a regulator, I had an awful time trying to
> get it to stop oscillating. I eventually discovered that this was
> because of the nonlinear behavior of the pass transistor at low
> currents (essentially because if the capacitance on the output got
> charged to even slightly too high a voltage and there was no load on
> the output, the op-amp's output would swing all the way to the
> negative rail untl the cap drifted down a bit in voltage).
>
> I've attached a small PDF of a circuit to illustrate this. An LTSpice
> simulation of this circuit shows it to be unstable when the current
> being drawn from the output jumps from 10mA to 1A.
>
> One way to mitigate this is to put a resistor in the feedback path and
> a capacitor directly from the op-amp output to its inverting input,
> thus slowing down the response. However, no matter how large I make
> this compensation capacitor, large enough output capacitor values will
> eventually cause instability. The same is true if I put a resistor on
> the output to draw a minimum load (to reduce the nonlinearity of the
> pass transistor at low currents).
>
> I've often heard of regulators being unstable with too little
> capacitance on the output, but I've never heard anyone complain of
> what happens when you have too much capacitance on the output.
>
> Is there something I'm missing here?
>
> Thanks,
>
> Sean
>
>
> --
> 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