Introduction:
A multivibrator is a type of electronic circuit that generates a continuous output signal with a specific waveform. It is commonly used in various applications such as timing circuits, frequency generation, and waveform generation. There are different types of multivibrators, including astable, monostable, and bistable multivibrators. In this case, we are discussing the astable multivibrator, which generates a square wave.

Astable Multivibrator:
An astable multivibrator is a free-running oscillator circuit that does not require any external triggering. It continuously switches between two stable states, producing a square wave output. This circuit consists of two cross-coupled amplifiers (usually transistors) with positive feedback.

Working Principle:
The astable multivibrator operates based on the principles of positive feedback and regenerative switching. Let's understand the working principle step by step:

1. Initial State: Assume one transistor is conducting (ON state) while the other is in cutoff (OFF state). The capacitor connected to the collector of the conducting transistor starts charging through the resistor.

2. Charging Period: As the capacitor charges, the voltage across it gradually increases. This causes the conducting transistor to enter the cutoff state, and the other transistor starts conducting. The charging process continues until the voltage across the capacitor reaches a certain threshold.

3. Discharging Period: Once the threshold voltage is reached, the capacitor starts discharging through the resistor connected to the collector of the newly conducting transistor. As the capacitor voltage decreases, the conducting transistor eventually enters cutoff, and the other transistor starts conducting again.

4. Cycle Repeats: The discharging process continues until the voltage across the capacitor reaches a lower threshold. At this point, the capacitor starts charging again, and the cycle repeats.

Output Waveform:
The astable multivibrator produces a square wave output, which has a constant amplitude and alternates between two voltage levels (high and low). During the charging period, the output is high (corresponding to the conducting transistor), and during the discharging period, the output is low (corresponding to the cutoff transistor). This continuous switching between high and low states generates a square wave.

Conclusion:
In conclusion, a multivibrator is a circuit that generates a specific waveform. In the case of an astable multivibrator, it produces a square wave output. The working principle involves the continuous switching between two stable states, resulting in the generation of a square wave with a constant amplitude and alternating voltage levels.