Wien Bridge Oscillator Design

  
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One of the simplest sinewave oscillators is the Wien Bridge Oscillator. Any circuit requires 2 conditions to oscillate. Tracing the path from the input, round the feedback network, back to the input there must be an overall phase shift of 0 degrees at one particular frequency. In other words, any signal travelling around this loop, must be in phas
Wien Bridge Oscillator Design - schematic

e with the original signal as it arrives back at the input and thus add to the input signal. As the signal travels around the loop, there will be a loss in the system (heat dissipation in the components, losses in the amplifier etc). Therefore there must be some form of gain in the loop, such that the signal arriving back at the input (having travelled around the loop) is larger than the original signal. If these 2 conditions are met, the oscillations will be sustained. FIG 1 shows a typical Wien bridge oscillator. The circuit relies on the series RC network (made up of R1 and C1) and the parallel RC network (made up of R2 and C2) causing a phase shift of 0 degrees at one particular frequency at the non inverting input. Thus from what we have said, taking the signal at the + input, through the op amp, through R1 and C1 and back to the + input causes a phase shift of 0 degrees at one particular frequency. The following involves some equations that look a bit hair raising, but if you compare FIG1 with a normal non inverting op amp circuit and have a rough grasp of how capacitors vary their characteristics with frequency, you should be able to follow them. An easy way of analysing this circuit is to consider the gain and phase shift caused by the Wien bridge network (the series and parallel RC components) and ignore the other components in the circuit, as shown in FIG 2 The parallel combination of 2 resistors is equal to the...



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