Audio amplifier circuit and controlling method thereof

An audio amplifier circuit includes a pulse width modulation circuit, an auxiliary loop circuit, and a main loop circuit. The auxiliary loop circuit corresponds to a first variable resistance value and a first variable current value. The main loop circuit is coupled among the second node, an output terminal, and the first node. The main loop circuit corresponds to a second variable resistance value and a second variable current value. Under a condition that the auxiliary loop circuit and the main loop circuit are turned on, the second variable resistance value decreases and the second variable current value increases after the auxiliary loop circuit enters into a first control state such that the main loop circuit enters into a second control state, and the first variable resistance value increases and the first variable current value decreases after the main loop circuit enters into the second control state such that the auxiliary loop circuit is out of the first control state.