Oscillator fundamentals — must an oscillator have an external AC input to operate?

Introduction / Context:
Oscillators generate periodic waveforms autonomously by using feedback and energy storage (L/C or R/C) to sustain steady oscillations. Confusing oscillators with amplifiers that require driven inputs can lead to incorrect block diagrams and test setups.

Given Data / Assumptions:

• Linear or nonlinear feedback oscillator (e.g., Wien bridge, Colpitts, Pierce).
• Proper biasing and loop gain ≥ 1 at the desired frequency with correct loop phase.
• Thermal and noise sources provide startup perturbations.

Concept / Approach:
An oscillator requires no external AC input. Instead, it relies on satisfying Barkhausen criteria (loop gain and phase) so that inherent noise is amplified and shaped into a stable periodic signal. Some oscillators include automatic amplitude stabilization (e.g., lamp or AGC) to limit amplitude growth and fix the operating point, but external periodic drive is not part of normal operation.

Step-by-Step Solution:

Verification / Alternative check:
Disconnect the signal generator: a true oscillator continues producing its waveform; an amplifier or filter stops.

Why Other Options Are Wrong:

Common Pitfalls:
Forgetting bias or gain margin; neglecting amplitude stabilization leading to clipping; expecting oscillation when loop phase/gain conditions are not met.

Final Answer:
Incorrect — an oscillator is self-sustained and does not require an external AC input