A Wien bridge oscillator is a type of electronic oscillator circuit that generates a sine wave output. It was invented by Max Wien, and it's widely used in various applications, such as audio signal generation and frequency generation in electronic test equipment. The primary components of the Wien bridge oscillator circuit include resistors, capacitors, and an operational amplifier (op-amp).
Here's a step-by-step explanation of how a Wien bridge oscillator circuit operates:
Basic Configuration: The Wien bridge oscillator circuit consists of a feedback network that includes two resistors (R1 and R2) and two capacitors (C1 and C2) connected in a specific arrangement, along with an operational amplifier (op-amp).
Feedback Network: The two capacitors, C1 and C2, are connected in parallel with opposite polarities, forming a series combination with two resistors, R1 and R2. The feedback network forms a bridge-like structure, hence the name "Wien bridge."
Op-Amp Amplification: The op-amp (usually configured as an inverting amplifier) amplifies the voltage difference between its inverting (-) and non-inverting (+) inputs. The op-amp tries to maintain the same voltage at both inputs by adjusting its output.
Phase Shift: The key to the operation of the Wien bridge oscillator is the phase shift provided by the feedback network. The phase shift occurs at the frequency of oscillation (f_osc) such that the total phase shift around the loop is 0° (or an integer multiple of 360°) for the circuit to sustain oscillation.
Phase Shift Analysis: Let's analyze the phase shift in the feedback network. At low frequencies (below f_osc), the capacitors behave like open circuits, and the resistors R1 and R2 create a voltage divider, providing a phase shift of 0°. At high frequencies (above f_osc), the capacitors behave like short circuits, and the resistors R1 and R2 act as a voltage divider again, providing a phase shift of 0°.
Frequency Determination: The frequency of oscillation (f_osc) occurs at the point where the phase shift of the feedback network is 180° (π radians). This occurs when the capacitive reactance (1 / ωC) and resistive impedance are equal in magnitude. The formula for the frequency is:
f_osc = 1 / (2π * R * C)
where R is the resistance value of either R1 or R2, and C is the capacitance value of either C1 or C2.
Start-up: Initially, the circuit is powered up, and due to random noise or small imbalances, there might be a slight phase shift. However, the op-amp amplifies this difference, and the feedback network adjusts to provide a phase shift of 180° at the oscillation frequency.
Sustained Oscillation: Once the circuit reaches the condition of 180° phase shift at f_osc, the feedback will be positive, and the oscillation will be sustained. The op-amp output will swing back and forth, generating a sinusoidal waveform at the frequency determined by the Wien bridge components.
It's worth noting that a Wien bridge oscillator requires careful design and component selection to ensure stable and accurate oscillation at the desired frequency. The oscillation amplitude can be controlled by adjusting the gain of the op-amp or by using amplitude control circuits in conjunction with the oscillator.