The term Bipolar is used to describe either a DC power supply with separate positive and negative voltage outputs, or circuitry which operates on a Bipolar DC power supply.
The majority of electronic circuitry operates on a single DC power supply voltage (typically positive). This helps reduce cost and simplifies circuit design in most cases. Examples include computer logic circuitry and the motors in an automobile like the starter motor or the power windows.
Early amplifier designs operated on single DC supplies, which required a method to “block” the DC voltage from reaching the input or output, or being passed between stages within the amplifier. A capacitor would typically be used for this purpose, which was also referred to as a “coupling capacitor” when located between stages within the amplifier.
Use of coupling capacitors did not come without a cost to signal integrity, thus the incentive to develop an alternative. In contrast, amplifiers operating on bipolar DC power supplies can be designed without the need for coupling capacitors, and are thus referred to as Direct Coupled. This type of amplifier will amplify DC as well as the AC audio signals.
AC Modulation of DC
There is also the special case of an AC signal being superimposed on a DC voltage. In these cases the current only flows in one direction while the voltage varies by the same total voltage as the original signal. For example, an AC signal which varies between plus 1 volt and minus 1 volt would appear as a signal that varies between plus 3.5 VDC and plus 1.5VDC at the output of the DC level-shifting circuitry operating on 5VDC. The AC voltage variation is centered on one-half the DC voltage or 2.5VDC.
Historically this was the case in early audio amplifiers which operated on single-ended DC power supply. After the AC audio input signal passes through a capacitor, it is “centered” at approximately one-half the DC power supply voltage within the amplifier circuitry. This makes the clipping behavior of a single-ended amplifier different than that of a bipolar amplifier; it can be asymmetric.
In contrast to single-ended designs, an amplifier operating on a bipolar DC power supply can amplify AC audio signals without the need for DC level shifting. This type of amplifier is power from a positive and negative DC voltage with the same nominal voltage; for example most IC Op-amps operate on +/-15VDC. This type of audio amplifier will amplify any DC voltage present at the input and present it to the output; which means they can be used for DC level shifting.
In contemporary digital audio, DC level-shifting circuitry is used to allow signals to pass between converters operating on only positive DC voltages and audio amplifiers operating on bipolar DC power supplies. In AD converter units, the signal is typically bipolar at the input, and is converted to single-ended by the level shifting circuit to feed the input of the AD converter IC. In DA converter units, the single ended output of the converter IC is level shifted to bipolar for output. With careful design, the audio circuitry can remain Direct Coupled through the entire chain.