Wien Bridge Oscillator: Circuit Diagram, Working Principle, Frequency & Advantages

Introduction

The Wien Bridge Oscillator is one of the simplest and most widely used sine wave oscillators. It generates a sinusoidal output waveform by replacing the conventional LC tuned circuit with an RC network.

The oscillator derives its name from the Wien Bridge circuit, a frequency-selective form of the Wheatstone bridge. Due to its excellent frequency stability, low distortion, and easy tuning, the Wien Bridge Oscillator is commonly used as an audio-frequency oscillator.

Unlike the RC phase shift oscillator, the Wien Bridge Oscillator operates with zero phase shift at its resonant frequency. It uses a feedback network consisting of a series RC circuit and a parallel RC circuit with identical component values.

RC Phase Shift Network

The RC network consists of a series RC circuit connected to a parallel RC circuit. This configuration effectively combines a high-pass filter with a low-pass filter, forming a highly selective second-order band-pass filter with a high quality factor (Q) at the resonant frequency.

At low frequencies, the series capacitor (C₁) has a very high reactance and behaves like an open circuit, resulting in almost zero output voltage. At high frequencies, the parallel capacitor (C₂) has very low reactance and behaves like a short circuit, again producing negligible output.

Between these two extremes, there exists a specific frequency at which the output voltage reaches its maximum value. This frequency is known as the resonant frequency (f_r) of the oscillator.

Resonant Frequency

At the resonant frequency f_r, the phase difference between the input and output voltages is zero degrees. At this point, the capacitive reactance equals the resistance (X_C = R). Under this condition, the output voltage is maximum and is equal to one-third (1/3) of the input voltage.

Working of Wien Bridge Oscillator

In the Wien Bridge Oscillator, feedback is applied to both inputs of the operational amplifier. A resistive voltage divider formed by resistors R₁ and R₂ provides negative (degenerative) feedback to the inverting input, allowing precise control of the amplifier gain.

The positive feedback required for oscillation is provided through the Wien Bridge RC network, which consists of series and parallel combinations of resistors and capacitors. This RC network feeds the output signal back to the non-inverting input of the operational amplifier.

The RC network produces zero phase shift at only one specific frequency—the resonant frequency. At this frequency, the voltages at the inverting and non-inverting inputs are equal and in phase. This causes the positive feedback to cancel the negative feedback, resulting in sustained oscillations.

Condition for Oscillation

Since the RC network attenuates the signal to one-third of the output voltage, the amplifier must provide a gain of at least 3 for oscillations to start.

For a non-inverting amplifier, the voltage gain is given by:

A_v = 1 + (R₁ / R₂)

To maintain continuous oscillations:

A_v ≥ 3

Frequency Limitations

The Wien Bridge Oscillator is generally limited to frequencies below 1 MHz. Higher frequencies require special high-frequency operational amplifiers due to limitations in the open-loop gain of standard op-amps.

Advantages of Wien Bridge Oscillator

• Produces low-distortion sine wave output
• Excellent frequency stability
• Easy tuning using resistors or capacitors
• Simple circuit design
• Widely used in audio-frequency applications

Applications

• Audio signal generators
• Laboratory test equipment
• Function generators
• Oscillators in communication systems