It is a classic problem when designing with op amps: you need the output to go to zero, but — for most op amps — you can’t quite get down to the supply rail. If your power options are a positive voltage and ground, you can’t get down to zero without a special kind of op amp which might not meet your needs. The best thing to do is provide a negative supply to the chip. Don’t have one? [Peter Demchenko] can help. He uses a simple two-transistor multivibrator along with some diodes and capacitors to generate a minimal negative voltage for this purpose.
The circuit is simple and only produces a small negative voltage. He mentions that into a 910 ohm load, he sees about -0.3V. Not much, but enough to get that op amp down to zero with a reasonable load. Unlike other circuits he’s used in the past, this one is efficient. With a 5-volt input, it draws less than 1.5 mA.
His previous circuit used LEDs and photocells, which led to low noise but wasn’t the best for efficiency. However, it is still a clever idea.
There are many ways to get a negative voltage like this. If you have an RS232 interface anyway, you might be able to swipe some from the port or your own driver, although it might be noisier than you want.
We’ve written extensively about “the negative rail” and how to get it. Of course, you can always throw a module at the problem. What’s your favorite way of getting a negative supply?
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