Negative Op Amps

Note: This is a rewrite of a message I wrote to the Parallax Basic Stamp Mailing List, but it seemed generally interesting.

A "normal" op amp can't go right to either supply rail. So if you run an op amp at -5 and 5 V, your output might range from -4.8V to 4.8V. The more current you take from the output, the lower the voltage range is. So if those numbers are at 10mA, at 50mA, it might drop to, say, +/- 4.7V.

Now, the problem is, you want a single supply. But is that practical? That depends on what you are doing. They do make single-supply op-amps. These are especially designed to go to at least one rail (usually the - rail). So if you supply ground (0V) and 5V, the output could range from 0 to 4.8V (for example). What does that mean? Well suppose you are buffering a DAC or PWM output. The output only goes from 0 to 5V anyway. So you'll never need that negative output swing. If you know your output would always range from 1V to 4V, you might not even need a special op amp at all.

However, what happens if you are using the op amp as an audio amplifier with a capacitor coupling the input? Well, then you are in trouble because the input is going to have + and - components and you are going to throw away half of them (horrible distortion). So for that sort of thing, you have to have an op amp that can go + or - or a way to bias the input so that it is never negative (and you don't invert the signal).

So if you need that, the question becomes, "How do I get a negative voltage?" There are a few common ways of doing this:

1) Use 2 9V batteries. Ground the center. Use the 9V to power your 5V regulator and use +/- 9V on the op amp.

2) Use a dc to dc converter module to convert some voltage you already have to -5V. Power Trends has some switching regulators that take +5 and deliver -12 (with the magnetics built in). I haven't used them, but we use the 38V to 5V regulator module in a product and other than being unnecessarily large, it works OK. Maxim and others make chips for this, but they require external inductors (that I know of). The Power Trends part is a module like the Stamp -- not a true IC.

The simplest DC to DC converter is just an oscillator of some sort (555, RC and an inverter, etc.), and a capacitive charge pump switched with diodes (see You can also use a transformer/rectifier scheme. Look up DC to DC converter on the Web. Switching regulators can do it by manipulating charge/magnetic field. Maxim and others make "voltage inverters" that have all you need except the capacitors.

3) An ugly trick I have used before works if you have low power requirements on the op-amp. First make sure the op amp's inputs and outputs are DC isolated (capacitor coupling on all in and out). This isn't uncommon anyway. Then take a 9V battery and build a voltage divider (50%). Ground the center of the voltage divider. Now the battery terminals are at 4.5V and -4.5V (just don't ground the battery ground -- it isn't ground, it's -4.5V. The downside is that the divider draws power, and the load will "upset" the 50%. Still, I have used this to make a small circuit run off a single 9V battery with no extra DC to DC converters, etc. The whole thing was just a fraction bigger than a 9V battery by itself -- two jacks, two capacitors, the op amp, two resistors for power, and a few parts in the feedback loop. This is somewhat less ugly if you use more parts (see who describes it better than I can). If you need more power (grunt, grunt) look at -- a novel use of an LM386.