Op amp astable multivibrator oscillator circuit

- circuit, equations and design details for a simple multivibrator oscillator using a single op amp or operational amplifier.

Multivibrator oscillators are used in many electronics circuits and they are simple to construct. It is possible to construct them using a couple of transistors, but it is also possible to construct a very simple multivibrator oscillator circuit using an operational amplifier. The circuit can be used in a variety of applications where a simple square wave oscillator circuit is required.

The use of an operational amplifier integrated circuit is ideal from many viewpoints. Although circuits can be made using just two transistors, operational amplifiers are also very cheap these days, and there is often little to choose in terms of cost.

What is an astable multivibrator

An astable circuit is one that has two states and it is not stable in either. It continually switches from one state to the other. Suitably tailored in a circuit it can function as an oscillator, regularly switching from one state to another.

Within the circuit it is normal to use an RC element to determine the frequency of the astable multivibrator oscillator. LC elements can also be used but they are less convenient and more costly in view of the coil, especially as astable oscillators tend to be used for relatively low frequencies and the coils tend to be large for these frequencies.

Astable multivibrator applications

Although there are many forms of oscillator available, an astable multivibrator can be used as a simple, yet effective oscillator. They can be made using two bipolar transistors, but a more convenient method is to often to have an astable multivibrator using an op amp or operational amplifier.

As a result of their simplicity, astable multivibrators find applications in a variety of different aplications where square waves or timed intervals are required.

Although used less these days because there are often other oscillators incorporated within larger ICs or other techniques available, but they have been used in frequency dividers as it is possible to lock them to a frequency lower than a reference frequency. This principle was used in many items of electronic equipment from televisions where the line and frame oscillators were able to synchronise to pulses in the video signal, and oscillators in early electronic organs where a high frequency reference oscillator was used to synchronise the lower frequencies for the different notes.

Op amp multivibrator circuit

The astable multivibrator using an op amp comprises two main sections.

  • Feedback:   This section of the amplifier provides frequency dependent feedback and controls the rate at which the capacitor charges and discharges. It plays a major pert in determining the frequency of operation. It is governed by the capacitor C1 and the resistor R1. It is applied to the negative input terminal of the op amp.
  • Hysteresis:   The hysteresis section of the astable multivibrator is formed by the resistors R2 and R3. These resistors effectively form a Schmitt trigger and enable the circuit to switch between the two states at different voltages for the positive going and negative going voltages on the inverting input. The switch voltages determined by the resistors determine also have an effect on the frequency because it takes less time for C1 to charge to a lower voltage for switching than a larger one.

The overall astable multivibrator circuit can be seen below and consists of on operational amplifier, three resistors and a capacitor.

Operational amplifier multivibrator oscillator showing the feedback and the hysteresis resistors
Operational amplifier multivibrator oscillator

The time period for the oscillation of the astable multivibrator is provided by the formula:

The formula to determine the frequency of operation of the astable multivibrator oscillator

The values should be chosen to fall within reasonable bounds for successful operation of the overall circuit. Typically values of resistor above 100kΩ or so should be avoided, although sometimes values of up to 1MΩ may be seen. By keeping values to reasonable limits more reliable and predictable operation is likely to be achieved.

Circuit operation

To look at how the op amp astable multivibrator circuit works take a start point where the capacitor C1 is fully discharged and the output of the op amp is positive - it will actually be at its positive saturated level close to the positive voltage rail

The capacitor C1 then starts to charge up via the resistor R1. It rises asymptotically towards the positive saturation voltage. As the end connected to the output of the op amp is at the positive saturation voltage, +Vsat, and the rate is determined by the time constant of the combination of C1 and R1.

As the capacitor charges up the voltage rises and as the junction of the capacitor and resistor is connected to the inverting input, when this reaches a point where the circuit switching voltage (the voltage on the positive or non-inverting terminal) as determined by R2 and R3, the output changes from positive to negative, i.e. -Vsat. The voltage on the non-inverting input also changes at this point.

Operational amplifier multivibrator oscillator waveforms showing the capacitor voltage and the output voltage
Operational amplifier multivibrator oscillator

When the circuit switches the voltage on R1 is now negative and will cause the voltage on the capacitor to fall and this continues until the voltage on the inverting input reaches the new voltage on the non-inverting input, when the output will again switch and the cycle repeats.

Although many multivibrator circuits may be provided using simple logic gates, this operational amplifier multivibrator circuit has the advantage that it can be used to provide an oscillator that will generate a much higher output than that which could come from a logic circuit running from a 5 volt supply. In addition to this the multivibrator oscillator circuit is very simple, requiring just one operational amplifier, op amp, three resistors, and a single capacitor.

By Ian Poole

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