Electronic – Generating plus and minus voltages for op amp

operational-amplifierpower supply

I have what is probably a simple question. I have googled it, but the answer doesn't make sense to me.

I am learning about op-amps and creating all sorts of circuits with my nice cheap 10cent op amps. I always power these from my lab supply using +/- 5V or 15V and it works great on the solderless breadboard. Creating simple things such as simple multivibrators and so forth which would only cost a few cents.

Now I start to wonder how to power such devices from a single DC power supply, such as a 5V wall wart of which I have many lying around.

Here is what I've found.

OPTION 1:

Use a "rail splitter" such as TLE2426. But the cheapest one I can find on digikey in thru-hole form is $1.83 (qty=1). Really? More than ten times the rest of my bill of materials (1 op-amp plus a few resistors and capacitors to make a multivibrator, for instance).

OPTION 2:

Use another op-amp as shown in "Virtual Ground" section of this page:
http://www.swarthmore.edu/NatSci/echeeve1/Ref/SingleSupply/SingleSupply.html

OPTION 3:

I read somewhere online that you can buy single-supply op-amps. I searched digikey but failed to find one. It has a column called "Vsupply single/dual" but all the ones I clicked on, when I went to the datasheet, the numbers quoted in that column were the dual +/- voltages.

Conclusion

Note that for small one-off circuits I don't care about paying $2 for a solution, but surely Option 2 is not the only other solution? If so, then from now on I will only buy dual-style (two-per-package) op-amps, since it seems you always need one to create the virtual ground for the other. Surely that is not the only solution?

Furthermore, Option 2 can't supply much current, I think — does that mean it won't work in some instances? In which instances will I run in to problems with Option 2?

Best Answer

The cheapest solutions, assuming you only have one supply, are to redesign the circuit so it will work from a single supply or generate the negative supply. All monolithic op-amps that I know of will actually work on a single supply- very few actually have a ground pin, so they don't know the difference between +/-5V and a single 10V supply. They do know (and don't like) signals going below the negative (or positive) supply, or even approaching it in many cases. I suggest you look at the datasheet for the LM324 which is a so-called single-supply op-amp, so-called because the output swings close to the negative supply (with a load to the negative supply) and the input common mode range includes the negative supply. There are many examples of circuits operating from single supplies in the full datasheet. A rail splitter 'chip' is used in none of them, but occasionally an extra resistor or two is required.

For example, as you mention a multivibrator:

enter image description here

The LM324 at a penny or two per op-amp may not be as fancy as your expensive 10-cent op-amps, but it's ubiquitous. Many of the circuits are limited by the (frankly, not spectacular) performance of that particular op-amp but you can subsitute something better if you like.

That just isn't very elegant in some cases- especially when things are DC coupled. In such cases, you can produce a negative supply with a cheap switching regulator or a charge pump chip such as a 7660. If you have a clock available, a charge pump can be made with a digital output, two MLCC caps and a dual diode.

β€œAn engineer can do for a dollar what any fool can do for two.”

― Arthur Mellen Wellington