Op-Amp Relaxation Oscillator
A very simple kind of oscillator can be made by charging a capacitor through a resistor (or a current source), then discharging it rapidly when the voltage reaches some threshold, beginning the cycle anew. Alternatively, the external circuit may be arranged to reverse the polarity of the charging current when the threshold is
reached, thus generating a triangle wave rather than a sawtooth. Oscillators based
on this principle are known as relaxation oscillators. They are inexpensive and simple, and with careful design they can be made quite stable in frequency.
In the past, negative-resistance devices such as unijunction transistors and neon
bulbs were used to make relaxation oscillators, but current practice favors op-amps
or special timer ICs. The figure above shows a classic RC relaxation oscillator. The operation is simple: Assume that when power is first applied, the op-amp output goes to positive saturation (it's actually a toss-up which way it will go, but it doesn't matter).
The capacitor begins charging up toward V+, with time constant RC. When it
reaches one-half the supply voltage, the op-amp switches into negative saturation (it's a Schmitt trigger), and the capacitor begins discharging toward V- with the same time constant. The cycle repeats indefinitely, with period 2.2RC, independent of supply voltage. A CMOS output-stage op-amp was chosen because its outputs saturate cleanly at the supply voltages. The bipolar LMlO also swings rail-to-rail and, unlike CMOS op-amps, allows operation at a full f 15 volts; however, it has a much lower fT (0.1 MHz).
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