The Wien bridge oscillator is a type of oscillator circuit that generates sinusoidal or sine wave signals. It was first developed by Max Wien, and it is widely used in various applications such as audio frequency signal generation, waveform testing, and frequency standardization. The key feature of the Wien bridge oscillator is its ability to produce a sinusoidal output without the need for any external signal source.
The basic Wien bridge oscillator circuit consists of an operational amplifier (op-amp) and a feedback network composed of resistors and capacitors. The circuit topology is as follows:
Operational Amplifier (Op-Amp): The op-amp used in the Wien bridge oscillator is typically a high-gain, differential amplifier with two inputs (inverting and non-inverting) and one output.
Feedback Network: The feedback network includes two resistors (R1 and R2) and two capacitors (C1 and C2) connected in a specific configuration. This network is responsible for creating the necessary phase shift and gain required for oscillation.
The operation of the Wien bridge oscillator can be understood in the following steps:
Initial Conditions: Initially, there will be no output at the oscillator. The op-amp operates in its linear region, and the capacitor voltages are at zero (discharged).
Positive Feedback: The feedback network provides positive feedback to the op-amp, which means a portion of the output signal is fed back to the inverting input (negative feedback is also present). This feedback leads to an increase in the input voltage at the inverting input.
Phase Shift: The key feature of the Wien bridge oscillator is the phase shift provided by the feedback network. The phase shift is created by the combination of resistors and capacitors, and it is typically set to 180 degrees at the desired oscillation frequency. The phase shift of 180 degrees is essential for sustained oscillations.
Gain: The op-amp amplifies the input voltage, including the phase-shifted feedback signal. The gain of the op-amp and the attenuation in the feedback network are carefully chosen such that the total gain around the loop (including the positive and negative feedback paths) is equal to or greater than one (unity gain).
Oscillation Start: Due to the positive feedback and the phase shift, the output voltage of the op-amp starts to increase. The circuit tries to amplify the signal indefinitely, but practical limitations like the op-amp's power supply and output saturation prevent this.
Automatic Gain Control: The Wien bridge oscillator has an automatic gain control mechanism. As the output voltage approaches the maximum limit, the gain of the amplifier reduces, which prevents the output from exceeding the saturation level. This control mechanism helps maintain a stable output amplitude.
Oscillation Frequency: The oscillation frequency of the Wien bridge oscillator is determined by the RC time constants of the feedback network. The frequency can be adjusted by varying the values of resistors and capacitors in the network.
By ensuring that the total phase shift around the loop is 360 degrees (i.e., full circle), the oscillations sustain, and a stable sinusoidal waveform is generated at the output. However, in practical applications, some form of amplitude stabilization and frequency control may be employed to improve the performance of the oscillator.