Working principle of class AB speaker amplifier

1. Bias setting and crossover distortion elimination
The class AB amplifier sets the static bias current so that the output transistor is in a weak conduction state when there is no signal, thereby compensating for the high power consumption of class A and the crossover distortion problem of class B.
Bias voltage compensation: Introduce a diode, resistor or constant current source between the base of the transistor to provide a bias voltage of about 0.7V to ensure that the transistor is not completely turned off near the signal zero point.
Crossover distortion suppression: By turning on the transistor in advance, the signal discontinuity problem caused by alternating switches in the class B amplifier is eliminated, and the positive and negative half-cycle waveforms transition smoothly.

2. Switching of working modes
Small signal mode (class A characteristics): When the input signal is small, the transistor is continuously turned on, maintaining linear amplification and achieving low distortion.
Large signal mode (class B characteristics): When the signal amplitude increases, the transistor is only turned on in the part exceeding the half cycle, reducing static power consumption and increasing efficiency to 50%-70%.

3. Circuit design and implementation
Push-pull output stage: complementary symmetrical NPN and PNP transistors are used to amplify the positive and negative half-cycles of the signal respectively.
Temperature stability design: The influence of temperature changes on transistor parameters is offset by thermal elements (such as thermistors) or feedback circuits to avoid the problem of "heat runaway".
Single power supply optimization: some circuits are designed through capacitor coupling or voltage divider to achieve symmetrical output under single power supply and reduce costs.

4. Performance advantages
High fidelity and low distortion: combined with the linear advantages of Class A, the total harmonic distortion (THD) is lower than Class B (typical value <1%).
Efficiency improvement: compared with Class A (efficiency 20%-30%), Class AB efficiency can reach 50%-70%, which is suitable for medium and high power scenarios.
Wide applicability: compatible with multiple types of loads (such as speakers, headphones), suitable for home theater, car audio and other scenarios.