opamp relaxation oscillator 
Relaxation oscillator realized with opamp is given in figure bellow.
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_{CC}). Change of the output voltage level automatically trigger capacitor C charging or discharging process. Let assume that initially output voltage was high (+V_{CC}). Via voltage divider R_{2}R_{3}, noninverting input is on electrical potential of:
Let's denote
Inverting input is on potential:
Positive voltage on R_{f} start to charge capacitor C with time constant τ=CR_{f}. Voltage on capacitor increases from V_{CC}·β tending to reach +V_{CC}. However, once when capacitor voltage exceeds potential of +V_{CC}·β, inverting input is higher then noninverting, and output of opamp goes to V_{CC}. This charging process takes time of:
After V_{} exceed potential of V_{+}, output voltage goes to V_{CC}, starting discharging process. Discharging process lasts for:
Obviously, for symmetrical power supply of opamp (±V_{CC}), T_{1}=T_{2}, so duty cycle is always 50%, independently of R_{3} over R_{2} ratio. R_{3} over R_{2} ratio affect frequency, but as logarithmic function. Frequency of generated signal is:
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External links: Opamp Relaxation Oscillator on Wikipedia Opamp Relaxation Oscillator on All about circuits

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