Although Class G and Class AB amplifiers look similar, Class G uses two or more supply voltages while Class AB uses only one supply voltage.
When operating at low signal levels, Class G amps select a low supply voltage. As the signal level increases, these amps automatically select the appropriate supply voltage. Class G amps are more efficient than Class AB amps because they use the maximum supply voltage only when required; in contrast, Class AB amps always use the maximum supply voltage.