In a free-running (astable) multivibrator, what initiates the oscillations at power-up and sustains continuous switching without any external trigger?

Introduction:
An astable multivibrator is a classic oscillator that generates a continuous stream of pulses or square waves with no stable operating point. It is used in clock generation, flashing indicators, and timing circuits. The question probes understanding of what starts and sustains such oscillation in the absence of external triggers.

Given Data / Assumptions:

• A standard astable multivibrator using positive feedback and RC timing (implemented with op-amps, comparators, or cross-coupled transistors).
• Power is applied to the circuit, and no external trigger input is provided.
• Active devices and passive components are subject to unavoidable small variations and noise.

Concept / Approach:
Because the astable has no stable equilibrium, any infinitesimal disturbance pushes the output away from its metastable point. Thermal noise, device offset voltages, and component mismatches provide the initial nudge immediately after power-up. Positive feedback then drives the output toward one state while an RC network times the transition to the opposite state, creating ongoing oscillation without any external assistance.

Step-by-Step Solution:
At power-up, offsets and noise create a small voltage difference at the comparator or amplifier input.Positive feedback amplifies this difference rapidly, forcing the output to a rail.The timing capacitor charges or discharges through a resistor toward a reference threshold.When the threshold is crossed, the feedback polarity flips and the output switches to the opposite rail.This cycle repeats indefinitely, establishing a free-running oscillation without any trigger.

Verification / Alternative check:
In practice, even perfectly symmetrical designs begin oscillating due to microscopic asymmetries or noise. Simulation with a tiny initial condition bias or noise source reproduces the same spontaneous startup behavior.

Why Other Options Are Wrong:

• A trigger / an input signal / an external circuit: These are not required for astable operation; they are used in monostable or externally clocked multivibrators.
• A one-shot pulse: Describes monostable startup, not an astable that runs continuously.

Common Pitfalls:

• Confusing astable (free-running) with monostable (one-shot) or bistable (flip-flop).
• Assuming a perfect balance would stop oscillation; in reality, offsets and noise always exist.
• Overlooking power-supply ramp behavior, which itself can provide the initial disturbance.

Final Answer:
nothing