Relaxation oscillator realized with op-amp is given in figure bellow.

Simulate Op-amp relaxation oscillator.

A relaxation oscillator is a circuit that combine Schmitt trigger with slow negative feedback realized with RC circuit. Output voltage oscillate from rail to rail (from **V _{CC}** to

**-V**). Change of the output voltage level automatically trigger capacitor C charging or discharging process.

_{CC}Let assume that initially output voltage was high (**+V _{CC}**). Via voltage divider

**R**, non-inverting input is on electrical potential of:

_{2}-R_{3}Let’s denote

Inverting input is on potential:

Positive voltage on **R _{f}** start to charge capacitor

**C**with time constant

**τ=CR**. Voltage on capacitor increases from

_{f}**-V**tending to reach

_{CC}·β**+V**. However, once when capacitor voltage exceeds potential of

_{CC}**+V**, inverting input is higher then non-inverting, and output of op-amp goes to

_{CC}·β**-V**. This charging process takes time of:

_{CC}After** V _{–}** exceed potential of

**V**, output voltage goes to

_{+}**-V**, starting discharging process. Discharging process lasts for:

_{CC}Obviously, for symmetrical power supply of op-amp (**±V _{CC}**),

**T**, so duty cycle is always 50%, independently of

_{1}=T_{2}**R**over

_{3}**R**ratio.

_{2}**R**over

_{3}**R**ratio affect frequency, but as logarithmic function. Frequency of generated signal is:

_{2}Go to op-amp relaxation oscillator.

**External links:**

Op-amp Relaxation Oscillator on Wikipedia

Op-amp Relaxation Oscillator on All about circuits

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