Electronic – Single supply inverting amplifier

inverting-amplifieroperational-amplifierphotodiode

I am using this circuit to measure the voltage of a small solar panel – around 3 or 4 V max (so instead of the photodiode consider there is a solar panel). That means Vin = -3 V. When I first built it, I didn't think about the fact that the op-amp is single supply and what effects that might have. Today, I was thinking about it and after reading some things, came to the conclusion that a single supply op-amp should not accept voltages outside its rail-to-rail range. I have 2 questions:

  1. Why is the anode of the panel connected to ground and not the other way around? (I based this circuit on one I saw online)

  2. Why does this circuit work? The output is exactly what I expected it to be in the first place (-0.65*Vin), but it shouldn't, because it's a single supply op-amp, right?

Thanks

schematic

simulate this circuit – Schematic created using CircuitLab

Best Answer

The current produced by the photodiode in the presence of light is a reverse current not a forward current. This means current flows from R1 thru D1 to ground.

This means that to keep both input pins of the op-amp at the same voltage (e.g. 0V), the op-amp output has to rise above 0V to feed current into R2 thus keeping -Vin and +Vin at precisely the same potential\$^1\$. As more light hits the photodiode the op-amp output has to rise higher.

If the photodiode were the other way round photo-current would be traveling into R2 and this means the op-amp output has to provide negative voltage and, of course it can do this when it has a negative supply.

I'm assuming you are using rail-to-rail op-amps in this analysis.

Same problem with solar cells - you need the left hand end of R1 to be pulled negative by the solar cell for this to work.

\$^1\$ If you don't understand why -Vin = +Vin then this is another question and is unrelated to photo-diodes or solar cells.