domingo, 7 de febrero de 2010

Amplificador Operacional como multivibrador

Operational amplifier multivibrator oscillator circuit

Multivibrator oscillators are used in many 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.
The operational amplifier multivibrator circuit comprises two sections. The feedback to the capacitor is provided by the resistor R1, whereas hysterisis is provided by the two resistors R2 and R3.

Operational amplifier multivibrator oscillator

Operational amplifier multivibrator oscillator

The time period for the oscillation is provided by the formula:

T = 2 C R1 loge (1 + 2 R2 / R3)

Although many multivibrator circuits may be provided using simple logic gates, this 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.

Operational amplifier bistable multivibrator

It is easy to use an operational amplifier as a bistable multivibrator. An incoming waveform is converted into short pulses and these are used to trigger the operational amplifier to change between its two saturation states. To prevent small levels of noise triggering the circuit, hysteresis is introduced into the circuit, the level being dependent upon the application required. The operational amplifier bistable multivibrator uses just five components, the operational amplifier, a capacitor and three resistors.

Bistable multivibrator operational amplifier circuit

Bistable multivibrator operational amplifier circuit

The bistable circuit has two stable states. These are the positive and negative saturation voltages of the operational amplifier operating with the given supply voltages. The circuit can then be switched between them by applying pulses. A negative going pulse will switch the circuit into the positive saturation voltage, and a positive going pulse will switch it into the negative state.

Waveforms for the bistable multivibrator operational amplifier circuit

Waveforms for the bistable multivibrator operational amplifier circuit

It is very easy to calculate the points at which the circuit will trigger. The positive going pulses need to be greater than Vo-Sat through the potential divider, i.e. Vo-Sat x R3 / (R2 + R3), and similarly the negative going pulses will need to be greater than Vo+Sat through the potential divider, i.e. Vo+Sat x R3 / (R2 + R3). If they are not sufficiently large then the bistable will not change state.
[1] Amplificador operacional como Multivibrador oscilador.
[2] Amplificador operacional como Biestable.

Gerald Soto, EES.

No hay comentarios:

Publicar un comentario