Op-Amp Pulse Generator Circuits

A number of operational amplifier circuits are provided here that are set up to be squarewave oscillators. A squarewave is a Periodic Wave pulse train with a 50 percent duty cycle. The operational amplifier is really just a high-gain amplifier, so oscillation is not a problem with just a little positive feedback [R4 provides the positive feedback

in the first schematic]. This style oscillator is formed by three separate pieces or functions; a comparator made up of the Op-Amp, an integrator formed by the capacitor and R1, and a latch formed by resistors R2 and R4 [which adds hysteresis to the comparator]. The amplifier oscillates based on the voltage applied to the V+ line, which is used to change the set point. Resistor R3 helps with bias current and could be removed. The exponential voltage which develops across the capacitor is converted to a current by resistor R1. The rising and falling exponential voltage is set by the product of the resistor capacitor combination. The frequency of oscillation is determined by the following equation: F = 0. 7 / (R1 x C). The period would than be 1. 4*(R1 x C). Because only one timing path exits, the capacitor changes and discharges though the same resistor, the circuit produces a square wave. the trip points, when the oscillator switches, is set by the comparator input which is determined by V+, R2 and R4. In most cases R2 is made equal to R4. This example of a square wave oscillator uses an LM124, a low power quad amplifier. Only one amplifier in the package is being used, so the other unused devices should be tied off. The output of an unused Op-Amp should be tied to the minus input, and the plus input should be left open [unconnected]. A rectangle wave is a squarewave with something other than a 50 percent duty cycle. A...

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