When T 1 is on, T2 is off, and the output of T2 collector is-voltage, and the square wave is high at this time;

When T 1 is turned off, T2 is turned on, the output of T2 is 0V, and the square wave is 0V. Convert the original square wave into a square wave of-15-0V.

Specifically, it is necessary to adjust the emitter resistance of T 1, T2 and the base bias potential of T2 to change the voltage amplitude.