Philip Pemberton wrote:
> I'm probably being really thick here,

Careful, you may not be allowed to say that anymore for fear you'd get upset
and say bad things to yourself ;-)

> As for the opamp and FET bit -- the 10-ohm resistor is (I assume)
> there to stop the opamp overdriving the gate on the FET.

No, the FET should be able to operate over the full 9V battery range, else
someone chose the wrong FET.  This schematic smells a bit like someone was
using rules of thumb and didn't stop to think what they were really for and
when they apply.  You often see gate resistors like R10 in fast switching
applications when the drain dV/dt is expected to be large and the circuit
can switch the gate quickly.  In this case the FET is being run in linear
mode and the opamp is likely not that fast, so that reason doesn't apply.

The FET gate does look like a capacitive load, and many opamps aren't stable
with those.  Some series resistance can keep the opamp stable in such cases.
I haven't looked at the LM358 datasheet, but 10 ohms sounds low for that
purpose.

> What I don't get is what R13 (and R14) are there for

Honestly, I don't either.  Again, this smells like someone copied parts of
this circuit from another purpose and may not have fully understood the
reason for every part.  Some opamps are more linear when forced to produce a
little output current, but 100Kohms sounds high for that purpose, and then
I'd tie it to ground instead of the top of the feedback resistor anyway.

Read the opamp datasheet very carefully, and if you don't find a legitimate
reason for R13, leave it off and see what happens.

One thing that bugs me about this circuit is lack of capacitive feedback to
aid with stability.  Again, I haven't looked up the LM358, but even a unity
gain stable opamp could run into trouble here.  A small cap between the
opamp output and its minus input would help stability, although slow
transient response too.  However, given the 47uF cap accross the input,
transient response is obviously not a big design criterion.


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