My colleagues and I are having a discussion about the circuit below, specifically: Is the op-amp being used as a comparator, or as an amplifier, or something else entirely?
The circuit is a negative voltage regulator that tracks a variable positive DC regulator. The LM337 (REG3) output is equal and opposite to the positive supply (connected to the top end of R5). The op-amp + input is referenced to GND. When the DC outputs are equal and opposite, the voltage divider R3-R5 produces 0V at the - input. The output of the op-amp produces the DC voltage required to put the LM337 regulator output at the desired voltage, exactly 1.25V more positive than the output (e.g. if desired output is -10V, ADJ is -8.75 because of the 1.25V reference voltage of the LM337).
Comparator? It's comparing the output to the reference. Amplifier? The LM337 is in the feedback loop and the op-amp is amplifying the error between the desired voltage and the reference. In other words, comparators are usually open loop, with rail-to-rail outputs, but this is definitely a linear circuit.