This is a voltage multiplier circuit. First circuit is used to double the a square wave (any amplitude). However there is minus about 2v losses in base-emitter of the transistors and the diodes in first circuit.
To work, second circuit must fall to nearly 0.4v and rise to at least 5.6v. to prevent short-circuiting and both transistors coming on at the same time, the rise and fall times must be very fast. The third circuit is used to double an AC voltage. The AC voltage rises “V” volts below the 0v rail and “V” volts above the 0v rail. [Source: talkingelectronics.com]