Electronics > Beginners
Lm339 gain adjustment
(1/1)
vis5254:
i use  a comparator lm339 . one of the input given at inverting terminal is below 5v , mostly 3.5v (obtained after full bridge rectifiying12v ac and reducing down 14v dc to 3.5 scale using voltage dividers - 8k2 , 2k2 , 10k2 .

i use a variable resistor to drop 5v into some 3.3v across non inverting terminal . when i kept a resistor 2k2 , diode and 10k as a feedback to inverting terminal , i found output of comparator went high to 4.8V across output .

how exactly did that output voltage got regulated to that range ?
golden_labels:
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.[/list]

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.
____
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.
Navigation
Message Index
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod