A dual supply to an op amp allows amplification of both plus and minus (bipolar) signals (such as a normal AC signal biased to ground). There are op amps available that have rail-to-rail inputs and outputs, and will work from a single supply but, of course, they will only amplify unipolar signals.
A dual supply to an op amp allows amplification of both plus and minus (bipolar) signals (such as a normal AC signal biased to ground). There are op amps available that have rail-to-rail inputs and outputs, and will work from a single supply but, of course, they will only amplify unipolar signals.
A bipolar signal is one that goes plus and minus. It's unrelated to to whether a transistor is a bipolar type (which means it has both N and P type of charge carriers).
For a single transistor to amplify a bipolar signal, the transistor must be biased into the active region. This is true for any type of transistor. The signal then must be capacitively coupled to allow the average signal level to be in the middle of the transistors active region.
Amplifiers that are DC coupled, such as op amps, generally have both P-type and N-type transistors in a push-pull arrangement at the output so they can output a bipolar signal.