On Mon, Mar 15, 2010 at 9:01 AM, Bob Blick <bobblick@ftml.net> wrote:
>
> 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.

For small current source which uses small MOSFET like 2N7002,
the 10 Ohm may not be necessary. But then here will talking about
a Power MOSFET (IRFP450), so it becomes necessary.

>> 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.
>

Typically for small current source, R13 (or R14) can be small value
or even a short (and R6/R12 are of bigger value). But here R13/R14 are
of bigger value, probably because of the small R6/R12 value.

One potential issue with the circuit is at low output current value,
then the common-mode voltage seen by the op amp is near
ground potential, a Rail-to-Rail type is required in this
application if no negative power supply is used and the requirement
of accuracy is higher. In this case, it may not matter as the
accuracy requirement is not that high.

-- 
Xiaofan http://mcuee.blogspot.com
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