> In case A, you're using the voltage divider relationship to divide the
> output voltage by some value, and feed that back into the op-amp.
> This causes the op-amp to try to compensate the output until the two
> inputs are equal - so in effect, the output is now a constant multiple
> of the other (non-feedback) input. Without R2 to ground, there's
> no voltage scaling, and you get a factor of 1

Presumably Jake has the op-amp with the -ve terminal at the top. If
you have a zener as feedback (a to -ve i/p, k to o/p) with -ve to 0V
through a resistor you have a positive voltage reference, the output
following and buffering the zener. Check out NatSemi AN31

http://www.national.com/an/AN/AN-31.pdf

and others. Google op-amp circuits

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist