Astable multivibrator — number of stable states: Does an astable multivibrator have two stable states, or does it have no stable states and continuously oscillate between two levels?

Introduction / Context:
Multivibrators—astable, monostable, and bistable—are foundational timing circuits. Each type is defined by how many stable states it possesses. This question asks whether an astable multivibrator has two stable states or none, which determines whether it oscillates on its own or requires triggering.

Given Data / Assumptions:

• Standard definitions of multivibrator types.
• Positive feedback timing network that charges and discharges to switch states.
• Idealized analysis without component tolerances considered.

Concept / Approach:
An astable multivibrator has no stable equilibrium. It repeatedly switches between two levels due to charging/discharging dynamics in the timing network, creating a free-running square wave. A bistable (flip-flop) has two stable states and requires a trigger to change state. A monostable has one stable state and one quasi-stable state, returning to stable after a pulse. Therefore, saying “an astable has two states” is incorrect because those two states are not stable; the circuit cannot rest in either.

Step-by-Step Solution:

Verification / Alternative check:
555 timer in astable mode auto-oscillates; in bistable mode it latches; in monostable mode one-shot pulses occur—textbook confirmations of the state definitions.

Why Other Options Are Wrong:

• Correct: Not correct; two stable states belong to bistable devices.
• True only with positive feedback removed: Removing positive feedback will not produce two stable states characteristic of a bistable; it typically halts oscillation without forming a latch.
• True only when supply ripple is zero: Supply ripple does not change the state-count definition.

Common Pitfalls:
Equating “two output levels” with “two stable states”; overlooking the role of triggering in monostable and bistable circuits.

Final Answer:
Incorrect