Difficulty: Easy
Correct Answer: Positive feedback
Explanation:
Introduction / Context:
Oscillators are feedback amplifiers engineered so that, at one frequency, the fed-back signal reinforces the input and sustains a waveform without an external drive. The Wien-bridge oscillator uses an RC lead-lag network to feed a selected frequency back to the noninverting input of an amplifier. Understanding which kind of feedback creates the oscillation mechanism is crucial for correct design and troubleshooting.
Given Data / Assumptions:
Concept / Approach:
To meet Barkhausen at the oscillation frequency f0, the loop must deliver a net phase of 0° and loop gain of 1. The role of the Wien network is to select the frequency and provide positive feedback of that component to the amplifier input. Additional circuitry often introduces slow, nonlinear negative feedback to regulate amplitude (so distortion remains low), but this auxiliary path does not create the oscillation—positive feedback does.
Step-by-Step Solution:
Verification / Alternative check:
Practical designs (e.g., lamp-stabilized Hewlett–Packard 200 series) show noninverting gain ~3 with bridge attenuation 1/3, yielding loop gain ≈ 1 and stable sinusoidal output—an explicit positive-feedback arrangement.
Why Other Options Are Wrong:
“Negative feedback” alone would not sustain oscillations. “Both with equal roles” misstates the primary role: positive feedback sets oscillation; negative feedback only stabilizes amplitude. LC tank networks are unnecessary; Wien oscillators are RC based. Parametric feedback is not used here.
Common Pitfalls:
Confusing amplitude stabilization (negative feedback) with the oscillation criterion (positive feedback).
Final Answer:
Positive feedback
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