A Wien bridge oscillator is a type of electronic oscillator circuit that generates sinusoidal or sine wave signals. It's based on the principle of feedback, where a portion of the output signal is fed back to the input with a phase shift and amplification. The circuit is named after its inventor, Max Wien.
The key components of a Wien bridge oscillator circuit include resistors, capacitors, and an operational amplifier (op-amp). Here's how the circuit operates:
Basic Structure:
The basic structure of a Wien bridge oscillator consists of a feedback loop where the output of an op-amp is fed back to its inverting (-) input through a network of resistors and capacitors. This network forms a bridge configuration.
Bridge Configuration:
The bridge network consists of two arms in parallel: one arm with a resistor (R) and a capacitor (C) in series, and the other arm with just a resistor (R). This configuration creates a voltage divider network that can be balanced under certain conditions, resulting in oscillation.
Frequency-Determining Components:
The frequency of oscillation in a Wien bridge oscillator is determined by the values of the resistors and capacitors in the bridge network. The ratio of the resistances and the ratio of the capacitances between the two arms should ideally be the same for oscillation to occur.
Positive Feedback and Phase Shift:
The op-amp amplifies the difference between its non-inverting (+) and inverting (-) inputs. The feedback network provides a phase shift of 180 degrees at the desired oscillation frequency. This phase shift, along with the additional 180 degrees of phase shift introduced by the op-amp itself, results in a total phase shift of 360 degrees, effectively providing positive feedback for oscillation.
Start-up and Oscillation:
Initially, there may be noise or slight imbalance in the bridge network. The op-amp amplifies this imbalance, causing the output to grow. Due to the feedback and phase shift, the signal continues to reinforce itself until it reaches a stable sinusoidal oscillation.
Amplitude Control:
The amplitude of the output signal can be controlled by adjusting the gain of the op-amp or by adding amplitude stabilization circuits to the Wien bridge oscillator.
Automatic Gain Control (AGC):
A Wien bridge oscillator can be susceptible to amplitude variation due to changes in the gain of the op-amp or other factors. To address this, an AGC circuit can be added to maintain a more consistent output amplitude.
Frequency Tuning:
The oscillation frequency can be adjusted by changing the values of the resistors or capacitors in the bridge network. Typically, fine-tuning is achieved through variable resistors or capacitors.
In summary, a Wien bridge oscillator operates by utilizing positive feedback and phase shift to create self-sustaining oscillations at a frequency determined by the values of the components in the bridge network. This makes it a simple and widely used circuit for generating sinusoidal waveforms in various electronic applications.