The type of circuit most often used to generate square or rectangular waves is the multivibrator. A transistor multivibrator is basically two amplifier circuits arranged with regenerative feedback. One of the amplifiers is conducting while the other is cut off.
Multivibrators are classified according to the number of steady (stable) states of the circuit. A steady state exists when circuit operation is essentially constant; that is, one transistor remains in conduction and the other remains cut off until an external signal is applied. The three types of multivibrators are the astable, monostable, and bistable.
The astable circuit has no stable state. With no external signal applied, the transistors alternately switch from cutoff to saturation at a frequency determined by the RC time constants of the coupling circuits.
The monostable circuit has one stable state; one transistor conducts while the other is cut off. A signal must be applied to change this condition. After a period of time, determined by the internal RC components, the circuit will return to its original condition where it remains until the next signal arrives.
The bistable multivibrator has two stable states. It remains in one of the stable states until a trigger is applied. It then flips to the other stable condition and remains there until another trigger is applied. The multivibrator then changes back (flops) to its first stable state.