Hi guys,

I'm probably being really thick here, but I've analysed this circuit a 
couple of times (as an amplifier and as a comparator) and came up blank 
each time.

I'm playing around with the "dual current sink" from this article:
   http://www.edn.com/article/CA6451249.html
... but before I build one, I'd like to know how it works. The function 
of the circuit is obvious, but I'd like to know more about the "what and 
why".

First I stripped it down to the bare essentials -- eliminated everything 
in the top half, then reintroduce the 9V battery to power the second 
(lower) half/channel. Now remove the switch and pots, and leave the +in 
open (aside from R11 and C5). It appears that the resistor would serve 
as a pull-down to stop the opamp input floating to Vcc and Blowing Stuff 
Up (tm), and the capacitor appears to be there to deglitch the "current 
set" input.

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. R12 (the 1-ohm 
resistor) is there to act as a current shunt; the opamp compares this 
against the "current set" voltage and continuously adjusts the output 
such that Iout = the voltage on +IN, but in amps (2.5V = 2.5A).

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?

As I said above, this is probably a candidate for "stupidest 
opamp-related question in the universe" but I'd rather like to know what 
those two resistors are doing... The LM358 datasheet doesn't show 
anything even broadly similar as near as I can tell.

Thanks,
-- 
Phil.
piclist@philpem.me.uk
http://www.philpem.me.uk/
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist