try to look for error amp in an op-amp book. It should
have a diagram on how it should be used.

John

--- 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
> > -- 
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