My negative voltage rail doesn't work

[mheruian]

Hi everybody,

for some reason, i learn how op-amps work (e.g. non-inverting and inverting configuration), i even test them on a online simulator and it works. Its just that it doesn't work when i applied it why is that? If ever, please teach a beginner like me. I am no genius so a detailed layman's term answers would be so great

my setup:



my simulation:

[mheruian]

I even tried using two (2) separate +12v supply that were grounded to each other.

each +12v supply has one (1) +5v voltage regulator.

first +5v voltage regulator acts as a supply only on the op-amp.

second +5v voltage regulator acts as an input on the op-amp.

still, same results. The +5v input on inverting op-amp doesn't output -5v output. Why is that??

[oPossum]

You are expecting an opamp to output -5V when you have only supplied it with +5?

[mheruian]

Oh?? i see. thanks. so there is no way to create a negative supply using op amp?? 

[Muxr]

Yes you can create a bipolar power supply with an Op Amp. Basically you can use an Op Amp as a rail splitter in order to create a virtual ground.

The ground in the schematic is a virtual ground. This is the ground the circuit using this bipolar supply should reference. The old GND from the battery becomes -6V and the 12V rail becomes +6V.

[Jwillis]

Can I use a LM741 for this or does it specifically have to have Rail-to-Rail Input and Output capabilities specified in the data sheet?

[oPossum]

LM741 isn't a good choice for any application.

NJM4556A would be a reasonable choice for a rail splitter.

Another option for some applications would be an ICL7660A or similar charge pump voltage converter.

[Zero999]

LM741 isn't a good choice for any application.

That's a bold statement.

Why is the LM741 unsuitable as a rail splitter? It seems perfectly suited to the role, as long as the output current rating isn't exceeded. There are better op-amps, but it's cheap, widely available and will work.

[mheruian]

Thanks for all the reply i really appreciated it, i learned a lot. I had done it using 741.

[hsn93]

hello, im noob like you but i think you cant reach more than 35mA on both "+6v" or "-6v" rails (ambient temp)

if im mistaken anyone correct me?


and for my information,



when using this virtual ground does it mean that both sinking and sourcing transistors (Q14 and Q20) will be closed at the same time?

[mheruian]

Hi hsn93, i guess your right.

though my target why i want a negative voltage rail was to use it on another op amp as a -v supply. I guess 35mA is more than enough for that since if I am not wrong also, supply current of 741 is around 3mA right? noob for life me 

[james_s]

Why are you using two separate 7805 regulators? You have two identical +5V supplies referenced to the same ground, there is no difference here between this setup and using just one regulator and connecting the same output to both nodes. If you want a negative supply why don't you make a proper negative supply? If you have two isolated supplies you can wire them in series and use the midpoint as ground.

[mheruian]

Hi James thanks a lot for the response. I didn't know that would work out. Would try that. Thanks

[james_s]

It will work provided they are *isolated* supplies. For example if you have two separate "wall wart" type power supplies, or the ones that are like laptop chargers, something of that sort then you can wire them in series. Otherwise batteries also work. I've used a pair of 9V batteries in series to get a bipolar supply for playing with op amps.

[Audioguru]

when using this virtual ground does it mean that both sinking and sourcing transistors (Q14 and Q20) will be closed at the same time?

No. if they both conduct at the same time then they would be shorting the supply and they would burn out. They both conduct only a very small current when their output voltage is near half the supply voltage. Then when the output of the opamp needs to go positive, Q14 turns on and Q20 turns off (a door closes, a transistor turns off). When the output needs to go negative then the opposite occurs.

Lookup Class-AB Amplifier Output.

[Zero999]

hello, im noob like you but i think you cant reach more than 35mA on both "+6v" or "-6v" rails (ambient temp)

if im mistaken anyone correct me?


and for my information,



when using this virtual ground does it mean that both sinking and sourcing transistors (Q14 and Q20) will be closed at the same time?

Hi hsn93, i guess your right.

though my target why i want a negative voltage rail was to use it on another op amp as a -v supply. I guess 35mA is more than enough for that since if I am not wrong also, supply current of 741 is around 3mA right? noob for life me 

Yes the LM741 can't output more than 20mA or so, but that's not normally a problem, when it's used in a virtual 0V circuit.

The LM741's output only passes the current through the 0V terminal. The current flowing from the +V to -V terminal makes no difference to the LM741. It's only the difference in currents between the +V and -V rails which pass through the 0V terminal. If the current through the +V terminal is 110mA and the current through the -V terminal is 100mA, then 10mA will be flowing into the LM741's output and it will be fine.

[hsn93]

The LM741's output only passes the current through the 0V terminal. The current flowing from the +V to -V terminal makes no difference to the LM741. It's only the difference in currents between the +V and -V rails which pass through the 0V terminal. If the current through the +V terminal is 110mA and the current through the -V terminal is 100mA, then 10mA will be flowing into the LM741's output and it will be fine.

so this means that the load on negative rail should be almost equal to the load connected to the positive rail, if they Rload+ and Rload- values are too different this will burn the amp? right?

[Audioguru]

The design of the 741 opamp is 50 years old and it was designed to use ONLY a 30V supply (plus and minus 15V). Some of them work with a supply as low as 10V.
Like most opamps it limits the maximum shorted output current to 35mA when it has a 30V supply which is probably much less with a 6V supply, then it does not burn out. If the supply is 30BV and you short the output then it heats with up to 30V x 35mA= 1.05W which is more than double its maximum allowed heating of 500mW so then it will overheat and be damaged.