We are now shown the circuit schematic of an opamp circuit known as a relaxation

oscillator.

This circuit produces an output with no signal input.

The only input to this circuit is a DC voltage used to

supply power to this opamp.

When the power supply is turned on at the output, a square wave will be produced.

That oscillates between the V plus rail In the V minus rail of the opamp.

The frequency of the square wave is related to the time constant set by this

R and this C.

It's a simple circuit to build and

actually you could experimentally determine values here to produce

the square wave of the frequency that you desire.

And the square wave could be used say as the input to a MOSFET switch,

turning something on off on off.

But say instead of a square wave you wanted to generate a sinusoidal

output voltage.

Remember the square wave, we can think of it in terms of its spectrum,

where the square wave is composed of individual sine wave components.

Then say that we apply the square wave to the input of a band pass

filter where we filter out using an ideal band pass filter this component.

And eliminate these additional spectral components.

Well then at the output of the band pass filter,

because the spectrum now consist of a single spike on the frequency spectrum.

We know that it would be a sinusoidal wave form.

So, by band pass filtering a square wave, we can generate a sine wave.

If we band pass filtered out this component, we would generate a sine wave

at 3 kilohertz, rather than at the fundamental of 1 kilohertz.

If we filtered this one, we could generate a 5 kilohertz sine wave.