Transistor Class A Power Amplifier
The transistor is biased in class A. That’s mean the collector current flows all the time. This current can increase or decrease, caused by the input signal which increases and decreases the forward bias. This increasing or decreasing depends on the input signal, but never stop, even in the lowest level of input (the lower peak of the waveform). These changes in current in the primary of the transformer and the induce signal currents in the secondary.
The output impedance of the transistor is matched by the transformer to the loudspeaker impedance. High current even if there is no signal input is the disadvantage of this circuit. Using class B push-pull is more efficient.