Astability

Astability refers to a property of a dynamical system in which there is no stable equilibrium state. In such systems, trajectories do not settle at a fixed point; instead, the state continuously evolves, often producing oscillations or other ongoing motion. Astability typically requires continuous energy input or feedback that sustains motion.

In electronics, astable describes a circuit that has no stable resting state and therefore produces a continuous output waveform. The classic example is the astable multivibrator, a two‑transistor circuit in which each transistor alternates between saturation and cut‑off. Feedback between the stages, along with timing components such as resistors and capacitors, drives periodic switching and sets the period and duty cycle of the output. Variants include RC relaxation oscillators and single‑supply configurations using op‑amps or diodes; all produce a repeatable, non‑settling waveform, typically a square wave or pulse train.

Beyond circuits, astability in dynamical systems denotes the absence of stable fixed points in the system’s equations. All fixed points may be unstable, with trajectories pursuing persistent motion that may form a limit cycle or more complex behavior. This concept is often discussed in contrast to bistability, where two stable states exist, and monostability, where a single stable state dominates.

Design and analysis of astable systems focus on sustaining motion while managing sensitivity to component variations and disturbances. In circuits, the timing values of resistors and capacitors determine frequency characteristics; in other domains, time delays, feedback strength, and nonlinearity govern the system’s oscillatory behavior.