astabile

Astabile is a term used in electronics to describe a circuit or state that has no stable operating point. In Italian, "astabile" means unstable. The most common reference is the astable multivibrator, a type of oscillator that continuously switches between two unstable states, producing a periodic output, typically a square wave.

In a classic two-transistor astable multivibrator, two transistors are cross-coupled through capacitors and resistors. Each transistor's collector feeds the base of the other via a capacitor; resistors provide base bias. When power is applied, one transistor turns on, discharges one capacitor, and drives the other transistor to turn off. The process then reverses, and the cycle repeats. The circuit thus has no stable state and will generate a steady oscillation. The output is typically taken from the collectors and can be shaped or buffered for further use.

Common implementations and variations include discrete transistor configurations, operational amplifiers, or dedicated timer ICs such as the 555 timer in astable mode. The oscillation frequency is determined by the RC network in the feedback paths (the charging and discharging time constants), with symmetrical components simplifying the timing calculation in many designs.

Applications of astable circuits include clock generation, basic waveform generation, tone generation in audio equipment, and timing references in digital systems. They are contrasted with bistable circuits, which have two stable states, and monostable circuits, which have a single stable state and require an external trigger to switch.