This complementary push-pull circuit is Class A and Class B. When the signal is small, it is in Class A, and when the signal is large, it is transferred to Class B..

In static state, Uo=0V, at this time Ub 1 is about 0.7V, Ub2 is about -0.7V, and T 1T2 are all in conduction state, but the conduction angle is small, the collector current is small, and the top and bottom are equal (blue line), so no current flows through the load RL, so no matter how the signal size changes after Uo = 0V, this static state.

When Ub 1↑(Ub2 equivalent synchronization ↑), the collector current of T 1 increases, and the increased part flows out of the load (red line); I feel that it is a single pipe work at this time, which is a class B;

When Ub2↓(Ub 1 equivalent synchronization ↓), the collector current of T2 increases, and the increased part flows out of the load (yellow line); I feel that it is a single pipe work at this time, which is a class B;