MULTIVIBRATOR & THEIR CLASSIFICATIONS

MULTIVIBRATOR

A multivibrator is basically a two-stage R-C coupled amplifier with positive feedback from the output of one amplifier to the input of the other. Multivibrator is a switching circuit and may be defined as an electronic circuit that generates nonsinusoidal waves such as rectangular waves, sawtooth waves, square waves, etc. Multivibrators are capable of storing binary numbers, counting pulses, synchronizing arithmetic operations and performing other essential functions used in digital systems. 

The circuit operates in two states viz. on and off controlled by circuit condition. The operation of the circuit is such that when one amplifier is cut off (off) the positive feedback loop maintains the other amplifier in a conducting or on the state. When a trigger causes one amplifier to change state, the coupling network acts to change the state of the second amplifier. The outputs are opposite in sense, one indicating a transition from an "off" state to an "on" state and the other the opposite transition. The condition in which the multivibrator may remain indefinitely until the circuit is triggered by some external signal is termed as a stable state.

There are only two possible states of a multivibrator and are as follows :

First state: Transistor Q¹ on and transistor Q² off.

Second state: Transistor Q¹ off and Transistor Q² on.

Depending upon the type of coupling network employed, the Multivibfiltons ale classified into the following three categories.

1. Astable or free-running multivibrator.

2. Monostable or single-shot multivibrator.

3. Bistable of flip-flop multivibrator.

The first one is the non-driven type whereas the other two are the driven type (also called the triggered oscillators).

Multivibrators are used for various purposes such as the generation of nonsinusoidal waveforms (square, rectangular sawtooth, etc.) and pulses occurring periodically, frequency division, synchronized generation of pulses and extended waveforms, generation of time delays, storage of binary bit of information, etc.

1. Astable or Free Running Multivibrator.

In an astable multivibrator, both coupling networks provide ac coupling through coupling capacitors. Each amplifier stage provides a phase shift of in the mid-band so as to provide an overall phase shift of 3600 or 00 and thus positive feedback. The circuit, therefore oscillates, provided that the total loop gain is equal to or greater than unity. It has no stable state. The two states of operation of astable multivibrator are quasi-stable (temporary) states. The astable multivibrator, therefore, makes successive transitions from one quasi-stable state to the other one after a predetermined time interval, without the aid of an external triggering signal. The periodic time depends upon circuit time constants and parameters. Thus it is just an oscillator as it does not need any external pulse for its operation. Since its output oscillates in between off and on states freely, it is called a free-running multivibrator. It is also named as square-wave generator based on its application. AMV may also be used as a synchronized oscillator and for driving sweep generators.

2. Monostable or Single-Shot Multivibrator.

It is also called a single swing, or delay multivibrator. In this circuit, one coupling network provides ac coupling while the other provides dc coupling. It has only one stable (standby) state and one quasi-stable state. In this circuit, a triggering signal is required to induce a transition from stable to quasi-stable state. The circuit remains in the quasi-stable state for a period determined by the circuit components. After this period, the circuit returns to its initial stable state and no external signal is required to induce this reverse transition. Thus the monostable multivibrator supplies a single output pulse of the desired time duration for every input trigger pulse.

3. Bistable or Flip-Flop Multivibrator.

The bistable multivibrator also called a two-shot multivibrator, requires the application of two triggers to return the circuit to its original state. The first trigger causes the conducting transistor to be cut off, and the second trigger causes a transition back to the conducting state. Because two triggers are required, bistable circuits are sometimes called flip-flops. Obviously, such a circuit does not oscillate. In this circuit, both coupling networks provide dc coupling and no energy storage element is used.

A bistable multivibrator is employed for performing many digital operations such as counting and storing binary information. This circuit also finds extensive use in the generation and processing of pulse-type waveforms.

Schmitt Trigger

Schmitt trigger and conventional bistable multivibrator may be used for the same applications. However, the conventional bistable multivibrator circuit, because of its symmetry, is preferred in applications that require the circuit to be triggered back and forth between two stable states. Schmitt triggers circuit is preferred wherever a free base terminal is required.

Important uses of a Schmitt trigger circuit are given below:

1. As a squaring circuit because it can convert inputs of any waveshape into output pulses of rectangular or square waveshapes.

2. As an amplitude comparator or level detector because in the Schmitt trigger circuit a change of state occurs whenever the input crosses a trigger point.

3. As a flip-flop circuit.

4. For reshaping the worn-out pulses by giving them sharp leading and trailing edges.