Class AB amplifiers combine Class A and Class B to achieve an amplifier with more efficiency than Class A but with lower distortion than class B.
This is achieved by biasing both transistors so they conduct when the signal is close to zero (the point where class B amplifiers introduce non-linearities). The transistors transition to class B operation for large excursions.
So, for small signals both transistors are active, acting like a class A amplifier. For large signal excursions, only one transistor is active for each half of the waveform, acting like a class B amplifier.
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