LM339 is a comparator, not a general purpose op-amp. It has no gain¹. Under conditions for which they are specified, comparators have only two stable outputs: high and low. LM339 has an open-collector output. Its “low” is around its negative rail (typically 500mV, depending on the current sinked). “High” simply means that the output is not sinking current at all — it may be treated as an open switch.
In that particular circuit:
- “High” is determned by the network formed of R4, D6, R5, R1 and R2.
- “Low” is around 0V — the exact value depends on how much current it has to sink.
One could drive the comparator so it never stabilizes, but this is not what they’re designed for. In particular open-collector output can’t source current. So while it may fall into oscillation, I doubt it can reliably produce nice, predictable results. But, if you really want to try: TI’s LM339 has amplification of at least 50000, typically 200000.
For the stable “high” and “low” states, see the attachment.
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¹ Not in the meaning used for op-amps. Technically the output stage must have some gain, but this is the same meaning of the word as in a logic inverter. As it happens, LM339 has a logic inverter as its output stage.