Electronic – Op-Amp comparator or amplifier

comparatoroperational-amplifier

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.

Negative tracking regulator

Best Answer

Is the op-amp being used as a comparator, or as an amplifier, or something else entirely?

It's being used as a linear amplifier I reckon and if you showed more of the circuit I think you'd find that the top of R5 goes to a positive power (or reference) rail. That positive power rail will dictate what the negative output voltage will be from the LM337.

Regulation occurs when the op-amp's inputs are equal and, because pin 3 is tied to 0V, when pin 2 attains 0V, the LM337 will be regulating.

So if the voltage at the top of R5 is +3.7 volts (pick any number really), the LM337 will output -3.7 volts.