LM358 strange behavior !

supply (there is a NPN transistor in the output stage). I had to build a mono-voltage subtractor (a voltage substractor built with a LM358 powered with

problem. I tried many other chip, it didn't change anything. I changed the 0v suply into a -12v one and the substractor worked well for every input voltage.

Le dimanche 4 novembre 2012 11:44:51 UTC+1, John Fields a écrit :

ve supply (there is a NPN transistor in the output stage). I had to build a mono-voltage subtractor (a voltage substractor built with a LM358 powered with 0v-12v supply) and with the consent of the electronic simulator ISIS I built (wired) such a subtractor. I was surprised that it could not entire ly do the job. For some subtractions, like 2v - 1.8v for example, the subs tractor said 0.62v instead of 0.2v. But for some other, it answered the rig ht thing, wich was 0.3v (a voltage lower than 0.62v) sometimes. So it was a ble to go under 0.62v !

ts problem. I tried many other chip, it didn't change anything. I changed t he 0v suply into a -12v one and the substractor worked well for every input voltage.

Ok. It is the basic soustractor schématic (not the instrumentation one wi th 3 amplifiers), with unity gain, visible here :

In my case, R1=R2=R3=R4=2.5 k, and Us = U2 - U1.

Michel.

e supply (there is a NPN transistor in the output stage). I had to build a mono-voltage subtractor (a voltage substractor built with a LM358 powered w ith 0v-12v supply) and with the consent of the electronic simulator ISIS I built (wired) such a subtractor. I was surprised that it could not entirel y do the job. For some subtractions, like 2v - 1.8v for example, the subst ractor said 0.62v instead of 0.2v. But for some other, it answered the righ t thing, wich was 0.3v (a voltage lower than 0.62v) sometimes. So it was ab le to go under 0.62v !

s problem. I tried many other chip, it didn't change anything. I changed th e 0v suply into a -12v one and the substractor worked well for every input voltage.

Your feedback resistors (R1-2) are asking the LM358's output to sink too much current. Close to ground, its ability is quite limited. See "Output Current Sinking Characteristics" on page 7 here:

The reason for the limitation can be understood easily by looking at the LM358 schematic on page 20--the output stage is an emitter follower in parallel with a current sink. Only

Cures: use larger resistors, or connect a load resistor or a current sink to the output.

supply (there is a NPN transistor in the output stage). I had to build a mono-voltage subtractor (a voltage substractor built with a LM358 powered with

problem. I tried many other chip, it didn't change anything. I changed the 0v suply into a -12v one and the substractor worked well for every input voltage.

Try putting a load R on the output, like a 10k or something in that range. You maybe in the cross over zone and that op-amp does not have much for shoot-through current.

Jamie

Oh, good for you! Recommending James Arthur's suggestion as your own is sheer genius.

Oops! In your vernacular, it should have been "Recommending James Arthur's sugesstion as your own is shear genus."

I don't know what the f*ck you're talking about, I have no idea who James arthur is and I don't give a s*it. Maybe you got some infatuation with him, I don't, who ever he is.

The fact that a 324, 358 having those issues being common knowledge as it is, leads me to believe that you're just a sit in stooly. Yes, one that is so full of s*it that I smell it all the way over here.

Oh, btw, incase you want to expand that fake no hands on knowledge of yours, a 358/324 etc can also oscillate if the output is brought down to or near zero output with no load on it.

What a piece of s*it you are. You belong with Phil, down under, the table, that is.

Jamie

Here is the message for Maynard:

Subject: Re: LM358 strange behavior ! Date: Sun, 4 Nov 2012 04:06:54 -0800 (PST) From: snipped-for-privacy@yahoo.com Newsgroups: sci.electronics.design References: 1

supply (there is a NPN transistor in the output stage). I had to build a mono-voltage subtractor (a voltage substractor built with a LM358 powered with 0v-12v supply) and with the consent of the electronic simulator ISIS I built (wired) such a subtractor. I was surprised that it could not entirely do the job. For some subtractions, like 2v - 1.8v for example, the substractor said 0.62v instead of 0.2v. But for some other, it answered the right thing, wich was 0.3v (a voltage lower than 0.62v) sometimes. So it was able to go under 0.62v !

problem. I tried many other chip, it didn't change anything. I changed the 0v suply into a -12v one and the substractor worked well for every input voltage.

Your feedback resistors (R1-2) are asking the LM358's output to sink too much current. Close to ground, its ability is quite limited. See "Output Current Sinking Characteristics" on page 7 here:

The reason for the limitation can be understood easily by looking at the LM358 schematic on page 20--the output stage is an emitter follower in parallel with a current sink. Only

Cures: use larger resistors, or connect a load resistor or a current sink to the output.

supply (there is a NPN transistor in the output stage). I had to build a mono-voltage subtractor (a voltage substractor built with a LM358 powered with

problem. I tried many other chip, it didn't change anything. I changed the 0v suply into a -12v one and the substractor worked well for every input voltage.

amplifiers), with unity gain, visible here :

Not strange at all. The common mode input voltage, U is equal to 0.5 * U2. With the negative supply pin of the op-amp being at zero volts, many common op-amps do not operate with that input. Also, a number of common op-amps cannot drive the output so close to its negative supply. Now..if you use a properly selected rail-to-rail op-amp..

The LM358 has an input CM range that is guaranteed to go down to the negative rail over the full temperature range. One of the things that makes it so useful for an extremely cheap and common op-amp (it's half an LM324). So, it's not that.

The LM358 is guaranteed to drive the output within 20mV of the negative rail (5mV typically) over the full temperature range, with a

His R1-R2 combination presents 5k from -Vin to output, so he'd need to sink 320uA for his example (Vin+ = 2.0v, Vin- = 1.8v, Vout = 200mV).

R1=R2=100k would go a long way toward fixing that.

Which overwhelms the 50uA nominal internal sinking capacity.

Sure, and make the other two 100K as well to kill most of the offset due to bias current.

I suppose one could do something like this and keep the 2K5 resistors:

|\| -|-\ | >-----+---- -|+/ | |/| | | | | | 1M8 | ___ |/ +12 -|___|--| 2PC4081R |> | | === GND

LOL.

Beta limit! Beta limit!

But with a 2:1 beta spread, it's perfectly reasonable.

A depletion mosfet would be fun, too. Or a better opamp. Or a negative supply for a better opamp.

).

LOL!

I nearly suggested a JFET load, Vgs=0v. Bigger resistors was easier though.

snipped-for-privacy@yahoo.com a écrit :

Hey, a resistor to a negative supply would be nice too :-)

lf

t.

Sure, if you've got one. (a negative supply, that is.)

Bob Pease did a neat trick--or was it Jim Williams?--using the e-b junction of a powered optoisolator as a negative current generator. The phototransistor served as a solar cell, to pull an LM358 output all the way down. Cute.

Was joking of course. If you have a neg supply then just use it for the opamp.

I think it was Bob Pease but I doubt this has enough current for an LM358.

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Missed it--whoosh. I often don't use negative supplies for V- for other reasons, like, it might be dirty, or to save power, or reduce loading on V-, etc.

Could be. If an ordinary phototransistor isolator has a current transfer ratio of 20%, and the phototransistor has a hfe of ~100, that suggests i.b ~= 0.2%. So, 10mA into the IRLED could make 10 or 20uA.