Single power supply Op Amps

[ziplock9000]

Am I right in thinking that single power supply op amps still can't give you a -ve output if you give a negative input?

So if you power them with GND and +5v, with a signal of GND and 0.1V your output will always be >= GND?


Running the osc in multisim, it seems that way on an LM324AD (almost)
Yellow = input sine, Blue = output sine

[danadak]

If the part is powered between gnd and its Vcc/Vdd terminal, yes,
you cannot get a negative output out of the OpAmp.


Regards, Dana.

[awallin]

correct. If you want to amplify only AC signals you can bias the input to +5V/2 and AC-couple your input (and output).
Figure 1 over here for example: http://www.analog.com/en/analog-dialogue/articles/avoiding-op-amp-instability-problems.html

[jeroen79]

An opamp will not be able to output beyond it's supply rails.
How close it can get to the supply rails will be specified in the data sheet.

'Single' supply is not really true.
It will still have a positive and a negative supply, even if the negative supply happens to be GND.

[ziplock9000]

Thanks everyone. I thought it was too good to be true coming from dual supply op amps. Back to square one.

[rstofer]

This is well traveled territory.  I highly recommend "Op Amps For Everyone" as it deals extensively with single supply op amps.

http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf

[ziplock9000]

This is well traveled territory.  I highly recommend "Op Amps For Everyone" as it deals extensively with single supply op amps.

http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf

Thanks but that 464 page document is way over the top for the yes or no answer I needed.

[Ian.M]

Chapter 4: Single-Supply Op Amp Design Techniques only has 22 pages, and the third page says:

Use of a single-supply limits the polarity of the output voltage. When the supply voltage VCC = 10 V, the output voltage is limited to the range 0 <= Vout <= 10. This limitation precludes negative output voltages when the circuit has a positive supply voltage, but it does not preclude negative input voltages when the circuit has a positive supply voltage. As long as the voltage on the op amp input leads does not become negative, the circuit can handle negative input voltages.

That's less than 5 minutes reading to find the answer for yourself - far quicker than the usual forum reply.  Of course you have to know the book exists in the first place or have supreme adept Google-fu to find it.

@Rstofer: Thanks! Saved for future reference.

[Zero999]

Bias the op-amp so it sits at half the power supply voltage, at DC. All the signal paths to the op-amp are coupled using capacitors. You might get more advice if you post a schematic.

[Ian.M]

A very useful chip if you are working with single supply OPAMPs, especially in battery powered applications is the TLE2426 rail splitter.  It gives you a far 'beefier' 1/2 Vcc rail without the wasted current of potential divider based solutions.

[ziplock9000]

Chapter 4: Single-Supply Op Amp Design Techniques only has 22 pages, and the third page says:

Use of a single-supply limits the polarity of the output voltage. When the supply voltage VCC = 10 V, the output voltage is limited to the range 0 <= Vout <= 10. This limitation precludes negative output voltages when the circuit has a positive supply voltage, but it does not preclude negative input voltages when the circuit has a positive supply voltage. As long as the voltage on the op amp input leads does not become negative, the circuit can handle negative input voltages.

That's less than 5 minutes reading to find the answer for yourself - far quicker than the usual forum reply.  Of course you have to know the book exists in the first place or have supreme adept Google-fu to find it.

@Rstofer: Thanks! Saved for future reference.

Not for me, it would have taken longer and I needed a sanity check, hence the forum post which was answered quickly.

[ziplock9000]

A very useful chip if you are working with single supply OPAMPs, especially in battery powered applications is the TLE2426 rail splitter.  It gives you a far 'beefier' 1/2 Vcc rail without the wasted current of potential divider based solutions.

Very interesting, thanks. But am I right in thinking that can only be used at 80mA? I'd need something similar to my PSU which is up to 2A

[Ian.M]

You don't need much current at 1/2 Vcc as most OPAMPs have very limited output current capability - tens of mA sourced or sunk at best.   If you are driving a high current load, you need to add a separate output stage anyway, and nowadays, with increasing integration,  you might as well make it a linear full bridge if the load can be floating, otherwise its a whopping great big cap and AC couple it so the current doen't need to return to the 1/2 Vcc rail.

[ziplock9000]

Thanks. I think I have enough to chew on now and make some progress.