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Function Generators

Not all audio oscillators are designed to produce sine waves. A basic square- and triangle-wave generator. The circuit shown to the right produces both square waves and triangle waves. It uses an op amp integrator to produce a ramp in response to a fixed input voltage, which is produced by the left op amp. When the ramp reaches a certain voltage as set by the ratio of R1/R2, the left op amp will rapidly switch polarity, and the integrator will output a ramp changing in the other direction. The frequency of operation is set according to the expression:

fout 1  (  R2  )

4RtC R1

While this circuit is very simple and stable, it also has some drawbacks. A major one is that the squarewave amplitude is not controlled; it will saturate the left op amp for both the positive and negative polarities. This slows down switching, which distorts the waveforms and limits the maximum frequency of operation. We can clip the squarewave output to avoid saturating the op amp, but the result will almost certainly upset the symmetry of the output waveforms — the squarewave will become somewhat rectangular, and the rise and fall times of the triangle wave will be different.

We'll see a number of improvements on this basic design in later pages in the set.

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