Multivibrator (Astable) Amplifier (OPAMP) / Op-amp Multivibrator

The Operational Amplifier, or Op-amp for short, is a versatile device that can be used in a variety of different electronic circuits and applications, from voltage amplifiers to filters and signal conditioners. However, a very simple and extremely useful op-amp circuit based on any general purpose operational amplifier is the Unstable Op-amp Multivibrator.

In our tutorials on Sequential Logic, we have seen that multivibrator circuits can be created from special chips such as transistors, logic gates, or NE555 timers. We also found that the unstable multivibrator constantly switches between its two unstable state without the need for any external triggering.

But the problem with using these components to produce an unstable multivibrator circuit is that many snap-ins are required for transistor-based unstables. Digital instability is usually only available on digital circuits, and the use of 555 timers may not always give us a result. However, the Op-amp Multivibrator circuit can provide us with a good rectangular wave signal using only four components, three resistances and a timing capacitor.

The Op-amp Multivibrator is an unstable oscillator circuit that produces a rectangular output waveform using an RC timing network connected to the inverted input of the transactional amplifier and a voltage dividing network connected to the other inverted input.

Unlike monostable or bistable, the unstable multivibrator has two states, none of which are stable, since in any case the time spent constantly switches between these two states, which are controlled by charging or emptying the capacitor through resistance.

In the op-amp multivibrator circuit, the op-amp works as an analog comparator. An op-amp comparator compares the voltages on its two inputs and outputs positive or negative, depending on whether the input is greater or smaller than VREF from some reference values.

However, since the open loop op-amp comparator is very sensitive to voltage changes in inputs, the output can switch uncontrollably between the +V(sat) and negative, -V(sell) feed rails, regardless of the input voltage. It is close to the measured reference voltage, VREF.

The op-amp used in the multivibrator circuit to eliminate irregular or uncontrolled switching operations is configured as a closed loop Schmitt Trigger circuit. Consider the circuit below.

Op-amp Schmitt Comparator

Multivibrator
Op-amp Schmitt Comparator

The above op-amp comparator circuit is configured as a Schmitt trigger that uses the positive feedback provided by R1 and R2 resistors to create hysteresis. Since this resistant network is connected between the amplifier output and the inverted (+) input, when the Vout is saturated on the positive feed rail, a positive voltage is applied to the inverted input of op-amps. Similarly, when the Vout is saturated with negative feed rail, a negative voltage is applied to the inverted input of op-amps.

Since the two resistors are configured as a voltage dividing network along the op-amp output, the reference voltage 1 (Vref) will therefore depend on the fraction of the output voltage that is fed back into the non-inverted input. This feedback fraction is given β as follows:

Multivibrator

+V(sat) is positive op-amp DC saturation voltage and -V(sat) negative op-amp DC saturation voltage.

Then we can see that the positive or upper reference voltage, +Vref (that is, the maximum positive value for the voltage in the inverting entry), is given as follows: +Vref = +V(sat)β, while the negative or lower reference voltage (i.e. the maximum negative value for the voltage at the inverting entry) is given as follows: -Vref = -V(sat)β.

Thus, if vin exceeds +Vref, the op-amp changes the status and the output voltage drops to negative DC saturation voltage. Likewise, when the input voltage drops below -Vref, the op-amp switches once again switch to the state, and the output voltage returns from negative saturation voltage to positive DC saturation voltage. When switching between two saturation voltages, the amount of built-in hysteresis given by the Schmitt comparator is defined by the difference between the two trigger reference voltages: VHİSTEREZ = +Vref – (-Vref).

Rectangular Transform from Sinusoidal

Apart from an op-amp multivibrator, one of the many uses of the Schmitt trigger comparator is that we can use it to convert any periodic sinusoidal waveform into a rectangular waveform, provided that the value of the sinusoid is greater than the voltage reference point.

In fact, the Schmitt comparator will always produce a rectangular output waveform, regardless of the input signal waveform. In other words, voltage input does not have to be sinusoid. It can be any waveform or complex waveform. Consider the following circuit:

Sinusoidal – Rectangular Converter

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Sinusoidal – Rectangular Converter

Since the input waveform will be periodic and have an amplitude large enough from the reference voltage (Vref), the output rectangular waveform will always have the same period, T, and therefore frequency, ε as the input waveform.

By replacing resistance R1 or R2 with a ponciometer, adjusting the feedback fraction, β, and therefore the reference voltage value on the inverted input, we can cause the op-amp to change the state of each half-loop anywhere from zero to 90o. Vref as the reference voltage remained below the maximum amplitude of the input signal.

Op-amp Multivibrator

By replacing the sinusoidal input with an RC timing circuit connected to the op-amp output, we can take the idea of converting a periodic waveform to a rectangular output a step further. This time, instead of a sinusoidal waveform used to trigger the op-amp, we can use the capacitor charging voltage, Vc, to change the output state of the op-amp, as shown.

Op-amp Multivibrator Circuit

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Op-amp Multivibrator Circuit

So how does it work? First, let's say that the capacitor is completely empty and the op-amp output is saturated on the positive feed rail. The capacitor starts charging from the output voltage, Vout reflector, R at a rate determined by the C, RC time constant.

From our tutorials on RC circuits, we know that the capacitor wants to fully charge the Vout value (+V(sell) within five time constants. However, as soon as the capacitor charging voltage in the inverter (-) terminal of op-amps is equal to or greater than the voltage in the non-inverting terminal (the output voltage fraction of op-amps is divided between resistors R1 and R2), the output status will change and the opposite negative feed rail will be driven.

However, the capacitor, which happily charges towards the positive feed rail (+V(sat)," now sees a negative voltage of -V(sat) on its plates. This sudden reversal of the output voltage causes the capacitor to discharge towards the new value of the Vout at a speed re dictated by the RC time constant.

Op-amp Multivibrator Voltages

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Op-amp Multivibrator Voltages

When the op-amp inverter terminal reaches the new negative reference voltage, -Vref in the non-inverted terminal, the op-amp changes the situation once again and the output is applied to the opposite feed rail voltage, +V(sat). The capacitor now sees a positive voltage on its plates and the charging cycle restarts. Thus, the capacitor continuously charges and discharges, creating a fixed op-amp multivibrator output.

The period of the output waveform is determined by the feedback rate generated by the R1, R2 voltage divider network, which determines the RC time constant and reference voltage level of the two timing components. If the positive and negative values of the amplifier's saturation voltage have the same magnitude, it will be t1 = t2, and the expression that will give the oscillation period is as follows:

Multivibrator

Where: R Resistance is the Natural Logarithm of the C Capacitance, ln() feedback fraction, T is the periodic time in seconds and the oscillation Frequency in ε Hz.

Then from the equation above we can see that the oscillation frequency for an Op-amp Multivibrator circuit depends not only on the RC time constant, but also on the feedback fraction. However, if we used resistance values that give a feedback fraction of 0.462 (β = 0.462), the oscillation frequency of the circuit would be equal to only 1/2RC, as shown, because the term linear log is equal to one.

Op-amp Multivibrator Example

An op-amp multivibrator circuit is created using the following components. R1 = 35kΩ, R2 = 30kΩ, R = 50kΩ and C = 0.01uF. Calculate the oscillation frequency of the circuits.

Multivibrator
Multivibrator

Then the oscillation frequency is calculated as 1kHz. When β = 0.462, this frequency can be calculated directly as follows: ε = 1/2RC. Also, when the two feedback resistances are the same, that is, R1 = R2, the feedback rate is equal to 3 and the oscillation frequency is: ε = 1/2.2RC.

We can take this op-amp multivibrator circuit a step further by replacing one of the feedback resistors with a ponciometer to produce a variable frequency op-amp multivibrator as shown.

Variable Op-amp Multivibrator

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Variable Op-amp Multivibrator

By adjusting the central ponciometer between β1 and β2, the output frequency will vary in the following quantities.

Ponciometer in β1:

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Ponciometer in β2:

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Then in this simple example, we can produce a rectangular waveform with variable output from 100Hz to 1.2kHz, or a processal amplifier multivibrator circuit that can produce any frequency range we need simply by changing the RC component values.

Above we saw that an Op-amp Multivibrator circuit can be installed using a standard operational amplifier such as 741 and several snap-ins. These voltage-controlled non-sinusoidal relaxation oscillators are usually limited to several hundred kilo-hertz (kHz). Because the op-amp does not have the necessary bandwidth, but they still make excellent oscillators.