Not all audio oscillators are designed to produce sine waves.
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:
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.