Electronic – Single supply differential amplifier

diff-amp

I am wondering is it possible to design a differential (or instrumentation) amplifier that will operate from single supply?

I have a 2Vp-p and 0.2Vp-p sine wave signals that need to go thru the differential amplifier. They both swing positive and negative.

But currently for this project I have only positive power supply. I can add an inverting converter if necessary but would like to know if I can reduce the costs.

If it is possible, could you please show an example.

Thanks

Best Answer

Getting the input of a amplifier to work outside its supply voltage range is difficult, but there are possible ways around this:

  1. Is the signal AC? If so, capacitively couple it to something that floats around ½ the supply voltage.

  2. Is the signal floating (not ground-referenced)? This can be the case when coming directly from some tranducer, for example. In that case, tie the other end to something around ½ the supply voltage.

  3. Shift the voltage level. This could be done with a resistor divider to the positive supply, for example. That will also attenuate the signal a little, but that can be made up by increasing the gain a little. Alternatively, the signal can be shifted by using zener diodes or ordinary diodes forward biased.

    Both these schemes will add some offset error. Everything is a tradeoff.

  4. make a negative supply. It's not really that hard or expensive, and there are off the shelf chips just for this purpose. For just powering a opamp, this could be a cpacitive charge pump, which will generally be cheaper than a inductive switcher.

    If you already have something that produces a regular clock, a capacitive charge pump can be as easy as a couple of diodes and caps. I have a few times used the clock output of a microcontroller for this purspose, although then I buffered it with two transistors making a push-pull emitter follower.