1; When the input Vin is at a high level (1), T3 is turned off, T2 and T4 are turned on, and the output is at a low level (). The collector current Ic of T4 flows in from the load current, which is what we usually call load capacity.

2； When the input Vin is at the offset level (), T2 and T4 are turned off, T3 is turned on, the output is at the high level (1), and T4 has no collector current Ic flowing out. T3 collector current current flows to the load.

3； You should carefully analyze the working conditions of T2, and you will know why.