Designing a Wien Bridge Oscillator for Sine Wave Generation

August 28, 2020 Cadence PCB Solutions

Key Takeaways

  • There are many different oscillators for generating various waveforms that are available as IC or can be made from discrete components.

  • A Wien bridge oscillator is a simple circuit that can be set to continuous oscillation, which outputs a sine wave.

  • The Wien bridge oscillator acts as a useful reference oscillator for analog circuits, and the output signal can then be manipulated with other analog circuits.

Sathorn-Narathiwas Intersection in Bangkok

The output from your Wien bridge oscillator will be a smooth curve, just like the  Sathorn-Narathiwas Intersection in Bangkok.

There are many oscillator circuits you can use to produce a clean AC signal, arbitrary waveform, or standard waveform. These can all be useful alternatives to crystal clocks, which can radiate noise at multiple harmonics and can even act like center-fed patch antennas if not laid out properly. If you need an oscillator that can be simply turned on with DC power and does not output strong harmonics, one good option is a Wien bridge oscillator circuit.

Why use this particular type of circuit instead of another analog oscillator circuit for sine wave generation? If designed properly, you can produce a clean sine wave with very low total harmonic distortion. In addition, the output frequency can be easily adjusted by setting the values of two resistors and/or two capacitors in the circuit, the latter of which can be controlled electronically with a varactor diode. Here’s how you can create a simple yet powerful Wien bridge oscillator and simulate its behavior.

What is a Wien Bridge Oscillator?

A Wien bridge oscillator is a simple circuit for generating a sine wave. This simple circuit does not perform a conversion between one signal and another to provide a stable reference output waveform. Instead, this is a standalone oscillator that incorporates amplification with RC elements on a positive feedback loop to produce an output signal. The principle of operation in this circuit is deceptively simple and does not rely explicitly on resonance to produce an output sinusoidal waveform.

Wein Bridge Oscillator Circuit Diagram

The circuit diagram below shows a typical Wien bridge oscillator circuit built from an op-amp. A basic LM741 or LM358 op-amp IC can be used to construct a Wien bridge oscillator. The only inputs come from the power rails. As can be seen, positive feedback to the non-inverting input is provided through a series RC circuit which provides high pass filtration to remove any DC offset. There is also a resistor network to the inverting input which provides negative feedback to stabilize feedback to the input such that the input is always within the amplifier’s linear range.

Wien bridge oscillator smooth output schematic

Wien bridge oscillator circuit diagram.

In addition, as long as the amplifier in the circuit is run in the linear range, the output waveform will have very low harmonic distortion. In effect, the two feedback loops work together to stabilize the output at a specific frequency and amplitude. The positive feedback loop (the RC sections) creates a voltage divider, thus the gain at the non-inverting input is less than 1. R1 and R2 should be chosen such that the gain in the negative feedback loop is greater than 1:

Wien bridge oscillator gain condition

Wien bridge oscillator gain condition.

The reason for this condition is quite important and relates to the role of positive and negative feedback in these oscillators.

The Role of Feedback on the Output Frequency

The design of a Wien bridge oscillator is related to another important concept in electronics that often doesn’t get attention. This idea is feedback. Here, we’re not just referring to the mechanism that provides gain in an op-amp. We’re really talking about the role of positive and negative feedback in creating a growing or decaying oscillation. As you can see above, the only input to a Wien bridge oscillator is the supply voltage from the rails. So what causes the output oscillation to begin?

This goes back to the fact that there will always be some noise in an electronic circuit. If noise travels through a positive feedback loop with gain, the force opposing feedback weakens and the signal will begin to grow at a faster rate as the signal is amplified. For a negative feedback loop with gain, the signal traveling on the feedback loop will grow, but the rate at which the signal will grow decreases.

This should illustrate the role of the positive feedback loop in a Wien bridge oscillator. Thermal noise in the op-amp circuitry produces an output signal, which gets amplified along the positive feedback loop and eventually grows into a strong signal. This is compensated by gain on the negative feedback loop, thus we have the condition defined above.

Finally, the two legs of the RC circuit shown above need to ensure that only a single frequency receives high gain and is output from the oscillator. For the diagram shown above, this frequency happens to be the cutoff for the RC filter divider on the positive feedback loop:

Wien bridge oscillator output frequency

Wien bridge oscillator output frequency.

This just happens to be the frequency at which the RC legs in the positive feedback loop oscillate with zero phase shift with respect to the non-inverting input. As a result, the differential input on the op-amp damps further growth in the oscillator output, yielding a stable sine wave.

Simulations for Your Wien Bridge Oscillator

Simulations of your Wien bridge oscillator need to consider the following points:

  1. Verify the amplifier runs in the linear range.

  2. The output frequency should obey the equation shown above.

  3. The voltage loop into the inverting and non-inverting inputs should be in phase.

Because these characteristics are defined in the time domain, you would need to use transient analysis. You could also create an equivalent linear model for the op-amp to use in the frequency domain, giving a transfer function for the circuit. The RC legs in the positive feedback loop should be examined in the frequency domain to verify which frequency will experience zero phase shift with respect to the voltage across the non-inverting input.

When you need to design and simulate a Wien bridge oscillator or any other oscillator circuit, you need the best PCB design and analysis software to create your circuit diagram and evaluate its behavior. The front-end design features from Cadence integrate with the powerful PSpice Simulator to create the ideal system for circuit design and evaluation. PSpice includes a variety of simulation features for analyzing oscillator circuits in the time domain and frequency domain, allowing you to examine all aspects of circuit behavior.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

 

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