Gandledorf said:
Am I at least correct in the assumption that, where the signal (without noise) goes from 0 to X volts, after the biasing, it goes from 2.5-X/2 volts to 2.5 + X/2 volts?
Although I've just got back from the pub, and am not 100%, that sounds about right
However, it's not something you should be bothering about at all - you are trying to amplify the modulation on the IR signal - this is totally separate to any DC conditions on the opamp. So the small AC signal from the phototransistor is fed through the capacitor and amplified by the AC gain of the opamp. The AC gain is set by the feedback resistor (which is shown as a pot) and the resistor in series with the capacitor (standard non-inverting opamp gain formula).