On Mon, 15 Mar 2010 00:48:25 +0000, "Philip Pemberton" said:

> 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. 

It's the other way around, it isolates the opamp from the large amount
of gate capacitance of the MOSFET. Otherwise the opamp would probably
oscillate.

> What I don't get is what R13 (and R14) are there for -- the 100k 
> resistors that connect the output of the opamp to the negative input of 
> the opamp. They're not there for output feedback in the traditional 
> sense (the path from the shunt resistor does that), so the only function 
> I can think of for them would be that of a pull-down resistor. The only 
> remaining question is, why do that? The LM358 opamp can drive 
> bidirectionally, so what's the point?

Probably again, a stability issue. The LM358 has a nasty class-B output
stage and without the resistor the circuit probably suffered from
oscillation, or at the very least, ringing when hit with stepped changes
to its input. The resistor reduces the gain of the opamp.

Cheers,

Bob

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