I've read several articles about the application of OP-AMP negative feedback. As far as an application for positive feedback, I came across hysteresis and it makes sense.
Are other uses for positive feedback other than hysteresis?
I am also not sure why in some applications they use both the positive and negative feedback path?
I've leanred that if the net gain on the negative feedback path is more than the net gain on the positive feedback path, then it acts as an OP-AMP with negative feedback, and vice versa.
If we tend to get an overall negative feedback or an overall positive feedback, then what is the point of using both paths at the same time, at the first place?
Why do we even bother ourselves to use both paths, if at the end all we care about is to have either a net positive or negative feedback?
Best Answer
See my answer here for an example of why you might want to use negative and positive feedback at the same time. The 598.3K resistor in the positive feedback path maintains a constant current through the variable resistor and the negative feedback path determines the gain (output volts per ohm of resistance of the variable resistor).
To see how this yields a constant current, consider Scott's generalized NIC answer. This is also a NIC- creating a negative resistance to cancel out the incremental effect of the 100K resistor R2, so the current remains constant.
Looking into the non-inverting input of the op-amp (without R2 and the variable resistor connected, but with R4 in place), the resistance looks like:
Referring to the NIC schematic, we have:-
R1 = 100K || 25.68K = 20.43K R2 = 122.2K R3 = 598.3K
\$ R_{IN} = \$ - 598.3K \$ \cdot \$ \$ 20.43K \over 122.2K\$ = -100K, which exactly cancels out the effect of the 100K resistor R2 (R2 on the original schematic).