Single rail opamp fundamental flaw? - Page 1
Forum section: electronics / projects

author: BiOzZ
author_rank: Regular Contributor
timestamp: April 18, 2013, 02:51:16 am
content: me and a friend of mine were working on a power supply together for a future project (that i must make a post about here) and he said we need a -1V rail to make the op amps give accuracy (vcc is 12v)i dont know much about opamps but im quite sure most single rail opamps work fine from VCC to 0V with a single rail input but he says the resitence in the totem pole wont let the output hit zero ... i said that resistance would be to small to be noticeable than he pointed out the voltage drop in the feedback ... claiming this is a fundamental flaw in single rail opampsso my question is do you need a negative voltage to negate the (if any) positive voltage offset in a single rail opamp?in this project i humored him and chose a copper amp with the uneducated claim that there designed for this ... ill look up my facts later O0PS: the power supply were making has many positive and negative rails so adding in a negative rail wont be too difficult with a redesign but for future reference id like to know

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 02:56:17 am
content: He's right, many "single-rail" op amps won'tquitehit the negative rail, at least under certain load conditions. You shouldn't have to bias it down like that for a simple power supply, though (well, depending on which op amp it is).

author: BravoV
author_rank: Super Contributor
timestamp: April 18, 2013, 02:56:51 am
content: How much current the opamp was sinking ? You might need lower negative rail if its too much.Consult the opamp's datasheet on the sinking capability.

author: BiOzZ
author_rank: Regular Contributor
timestamp: April 18, 2013, 03:01:40 am
content: how much will it generally offset from the negative rail? i know its probably in datasheets but were using general component symbols at this time

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 03:04:36 am
content: Pick an op amp before you continue. There are vast differences between models. For a simple power supply, if you don't require high precision, I recommend the commonLM324. Look under "Application Hints" for information about use near ground.

author: BravoV
author_rank: Super Contributor
timestamp: April 18, 2013, 03:17:13 am
content: Quote from: BiOzZ on April 18, 2013, 03:01:40 amEvery op-amp's datasheet has the row called "Output Voltage Swing" and it depends on the load.Even its called single rail op-amp, it doesn't mean the op-amp capable of maintaining Zero volt at output while sinking huge current.Just see the output as a simple transistor that has a VCEsat or Mosfet with RDSon, you just can not sink lots of current while maintain the output voltage say at certain level that you want to, there must be a certain acceptable limit at certain current source or drain.

author: BiOzZ
author_rank: Regular Contributor
timestamp: April 18, 2013, 03:27:59 am
content: Quote from: c4757p on April 18, 2013, 03:04:36 ami would not make the leap to call it a high precision power supply but its more precise than the LM324 ... i would pick up something more stable and longer lasting ... it can be socketed but would be a BITCH to get in to and replacefor this project we can offset the voltage on the output inner-software but for future reference thank you guys allot ... this might also explain troubles with my constant load ...

author: free_electron
author_rank: Super Contributor
timestamp: April 18, 2013, 03:28:34 am
content: when are people going to learn that there is no such thing as single rail or dual rail opamp.An opamp has a pin to feed it electrons and a pin to return electrons. that's it. those are the two supply pins.there is an input stage that works in a specified range referenced to these two supply pins.there is an output stage that works in a specified range referenced to these two supply pins.anything else you have ever been told about symmetrical and asymmetrical stuff is nonsense. it doesn't exist. it is an abstract concept and simplification of something that doesn't need any simplification.

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 03:33:55 am
content: Quote from: free_electron on April 18, 2013, 03:28:34 amWhen the datasheet writers stop making blatant oversimplifications.

author: BravoV
author_rank: Super Contributor
timestamp: April 18, 2013, 03:37:15 am
content: Quote from: free_electron on April 18, 2013, 03:28:34 am..aka... marketing jargon, nothing else.If I'm not mistaken, the term "Rail to Rail" in op-amp's world is also copyrighted isn't it ?

author: BiOzZ
author_rank: Regular Contributor
timestamp: April 18, 2013, 03:48:07 am
content: Quote from: c4757p on April 18, 2013, 03:33:55 amdoes it grind anyone elses gears when a data sheet writer puts stuff like "ultra accurate!" "best in class" and "NanoSpeed technology (r) (tm) (c) (u)"like who the hell do they think there marketing to?!"well i was going to go for the part with the specs i want ... BUT THIS ONE HAS NANOSPEED!" XPalso a side note i think single rail op amps are different than +/- rail onestho i know several dual rail opamps that work well with single rails ... like a TL082

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 03:52:41 am
content: Quote from: BiOzZ on April 18, 2013, 03:48:07 amAny op amp will work on a single rail system; as f_e said, there's really no difference. "Single rail" op amps usually can reach closer to the negative rail. I wouldneverdescribe a TL082 as working well on a single rail - the overhead it requires around both rails is quite large.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 18, 2013, 01:46:20 pm
content: if you search harder, you can find thread where our teks engineer revealed the reason why single or dual opamp terminology emerged, its much to do with historical reason. and no, its not oversimplification. its making a simple thing "complicated". a simple thing by nature, but complicated to convey to people's mind, what they did is try to simplify the explanation but in effect making the nature looks complicated.

author: free_electron
author_rank: Super Contributor
timestamp: April 18, 2013, 02:42:52 pm
content: @biozz : hahaa. Good one. Reminds me of bob peases fast and damn fast opamps. He actually got that datasheet published in a databook..Opamps are simple things. No need to make things complicated.