In Wein’s bridge, the output frequency is determined by

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RC combination

Explanation: The Wien's bridge oscillator is a frequency-selective bridge network that uses an RC series-parallel combination to determine the frequency of oscillation. The bridge achieves zero phase shift at a specific resonant frequency, which is determined solely by the values of the resistors and capacitors in the lead-lag network.

Working Principle: The frequency-selective feedback network consists of a series RC circuit and a parallel RC circuit connected in series. At the resonant frequency, the voltage across the parallel combination is exactly in phase with the input voltage, satisfying the Barkhausen criterion for oscillation. The bridge balance condition leads to the frequency expression where the reactive components cancel out.

Key Formulas: $f_0 = \frac{1}{2\pi RC}$ — Resonant frequency of the Wien bridge; $\beta = \frac{1}{3}$ — Feedback factor at resonance

Key Points: The Wien bridge is widely used in audio frequency oscillators.; It requires a minimum gain of 3 (A_v ≥ 3) from the amplifier stage to sustain oscillations.; The bridge utilizes a lead-lag network to produce zero phase shift.; Unlike LC circuits, RC networks are more suitable for lower frequencies.

Advantages: Excellent frequency stability; Easy to tune over a wide frequency range; Produces a low-distortion sine wave

Disadvantages: Requires a high-gain amplifier for oscillation; Frequency range limited by the values of R and C; Cannot generate very high (radio) frequencies effectively

Applications: Audio frequency signal generators; Distortion testing of audio equipment; Function generators

Additional Info: In the Wien bridge, the four arms consist of: one pure resistor, one pure capacitor, one series RC circuit, and one parallel RC circuit.. Option B (LC combination) is used in Hartley or Colpitts oscillators for high radio frequencies, not the Wien bridge.

Answer: C is correct — The output frequency of a Wien bridge is determined by the RC combination forming the lead-lag feedback network.

Exam Tip: Always remember that LC oscillators are for high-frequency (Radio) applications, while RC oscillators like Wien bridge and Phase-shift oscillators are for low-frequency (Audio) applications.

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