Wednesday, April 3, 2013

Using TLV2471 for Wein Bridge Oscillator

The Wien - Bridge oscillator is one of the simplest and best known oscillators and is used extensively in circuits for audio applications. The figure in the below shows the basic Wien bridge circuit configuration. On the positive side, this circuit has only a few components and good frequency stability. The major drawback of the circuit is that the output amplitude is at the rails, which saturates the op-amp output transistors and causes high output distortion.

The gain, A, of the negative feedback portion of the circuit must then be set. RF must be set to twice the value of RG to satisfy this condition. The op amp in the figure is single supply, so a dc reference voltage, VREF, must be applied to bias the output for full-scale swing and minimal distortion. Applying VREF to the positive input through R2 restricts dc current flow to the negative feedback leg of the circuit. VREF was set at 0.833V to bias the output at the mid rail of the single supply, rail-to-rail input and output amplifier, or 2.5 V, (see reference in the data sheet of oscillator). VREF is shorted to ground for split supply applications.

The Wien – Bridge oscillator circuit is shown in the figure, with component values selected to provide an oscillation frequency of 0 = 2f0, where f0 = 1/(2RC) = 1.59 kHz. The circuit oscillated at 1.57 kHz, caused by varying component values with 2.8% distortion. This high value results from the extensive clipping of the output signal at both supply rails, producing several large odd and even harmonics. The feedback resistor was then adjusted 1%. Figure 9 shows the output voltage waveforms. The distortion grew as the saturation increased with increasing RF, and oscillations ceased when RF was decreased by a mere 0.8%.

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