General Engineering Terms
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Squarewave Oscillator
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. |
 LM124 Squarewave Oscillator |
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].
Rectangle Wave Generator
A rectangle wave is a squarewave with something other than a 50 percent duty cycle. A pulse train is a term used when the duty cycle is either very high or very low, resulting in a recurring narrow pulse. This circuit is identical to the first oscillator except now there is an individual charging path for the capacitor and a discharge path that each use different resistor values.
 LM124 Pulse Generator | Dual Diode Variant Design Charging the capacitor is through the diode and Resistor R2, while discharging the capacitor is done through the opposite diode and resistor R1. The high portion of the output pulse is determined by C*R1, and the low portion of the pulse is controlled by C*R2. |
Single Diode Variant Design Yet another variation, but this time the circuit only requires one diode. Again, the LM124 is just an example one Op-Amp. The component values don't change based on which operation amplifier is being used. |
 LM124 Pulse Generator |
Circuit Drawbacks
These circuit designs operate off the [exponential] changing and discharging time of a capacitor and resistor. So care needs to be taken when selecting timing values and voltage trigger points, as the charging time is non-linear.
Additional Designs
Another operational amplifier designed as a square wave oscillator is the LM386, a low power audio amplifier [shown in the right side-bar]. The schematic is provided because it differs from the other examples, but it was extracted from some data sheet just like the other circuit examples. No details are provided, other than example component values and the output frequency.
It should be noted that these circuit designs have been around since the 1970's and published in many different books and application notes. The general design works for almost any operational amplifier, so what Op-Amp data sheet they end up in is just random chance, or what company data sheet they end up in.
Operational Amplifier Circuit Designs
LM386: Audio Mono Amplifier
LM741: Audio Stereo Amplifier
LM149: Audio Tone Control Filter
LM301: Audio 2-Band Tone Control Filter
TL082: Audio 3-Band Tone Control Filter
LM741: Audio Midrange Control Filter
LM381: Audio Mixer [Summer]
LM741: Audio Equalizer [Filter]
LM741: Resonant Equalizer [Filter]
LM741: All-pass Filter
LM101: Sine Wave Oscillator
LM107: Sine Wave Oscillator
LM108: Cosine Wave Oscillator
LM107: Triangle Wave Oscillator
LM3900: Triangle Wave Oscillator
LM143: Voltage Regulator
