Relaxation Oscillator

Theory


Relaxation Oscillator

A relaxation oscillator is a circuit that repeatedly alternates between two states at with a period that depends on the charging of a capacitor. The capacitor voltage may change exponentially when charged or discharged through a resistor from a constant voltage, or linearly when charged or discharged through a constant current source. With exponential charging, the timing is expressed in terms of time constants RC.



Fig.1 Shows Circuit Diagram of Relaxation Oscillator.

A classic op-amp relaxation oscillator is shown above. The non-inverting input is biased at V / 2, where V is the output voltage. The inverting input chases this value from below, and when it reaches it, the output voltage changes from one saturation level to the opposite one. The output is a square wave, with a frequency of 1 / 1.22 RC, and duty cycle of 50% if the saturation levels are symmetrical.
The inverting input and the output of the comparator are linked by a series RC circuit. Because of this, the inverting input of the comparator asymptotically approaches the comparator output voltage with a time constant RC. At the point where voltage at the inverting input is greater than the non-inverting input, the output of the comparator falls quickly due to positive feedback.
This is because the non-inverting input is less than the inverting input, and as the output continues to decrease, the difference between the inputs gets more and more negative. Again, the inverting input approaches the comparator's output voltage asymptotically, and the cycle repeats itself once the non-inverting input is greater than the inverting input, hence the system oscillates.


Fig.2 Square Wave Output.

A nonlinear or relaxation oscillator produces a non-sinusoidal output, such as a square, sawtooth or triangle wave. It contains an energy-storing element (a capacitor or, more rarely, an inductor) and a nonlinear switching circuit (a latch, Schmitt trigger, or negative resistance element) that periodically charges and discharges the energy stored in the storage element thus causing abrupt changes in the output waveform.