Multivibrator and its working principle

Multivibrator and its application

The oscillators which produce the output of square wave, rectangular sawtooth, triangle then, such types of oscillators are called multivibrators. They are used generate pulse such that the counting circuit, digital circuit, switching circuit, etc can be operated those types of multivibrators, the out is changed discontinuously. These are three type of multivibrators. They are given as below:

1. Astable mulitivirators
2. Monosatable mulivirators
3. Bistable multivibrators

Among these three, first (a) is not operated by another transistor.

• Astable multivibrator

In this astable multivirator, non of the state of output is stable. But both states are quasi-stable states. In this figure the output change between 5 V a d O V, the state of output changes without applying the external triggering (input), the time for each state s determined by RC component used in multivibrator circuit. An astablemultivibrator has no stable states and therefore changes back and forth between two unstable states without any external triggering. The timing capacitor C is charged towards +\(V_{cc}\) through resistors\(R_1\) and\(R_2\). The charging time \(t_1\) is given as,

$$ t_1 = 0.693(R_1 +R_2)C$$ The timing capacitor C is then discharged towards ground (GND) through resistors \(R_2\). The discharging time \(t_2\) is given as,

$$ t_2 = 0.693R_2C$$ The period T of the resulting clock waveform is the sum of \(t_1\) and \(t_2\). Thus

$$ T = t_1 + t_2 = 0.693(R_1 +2R_2)C$$ The frequency of oscillation is then found as

$$ f=\frac{1.44}{R_1+2R_2}$$

• Monosatable multivibrator

In the monostable multivribrator one output state is stable and other state is quasi-stable. The output always remains in stable state but when trigger is applied then the state changed to quasi-stable state. But after sometime the output returns to stable state. The time period of output in quasi-stable state is also determined by RC components used in multivibrators.

An external resistor R and capacitor C connected as shown in Figure are used to set up the 555 timer as monostablemultivibratorand its output is obtained as Figure. The pulse width of the output is determined by the time constant of R and C according to the following formula:

$$ t = 1.1 RC$$

• Bistable multivibrator

In bistable multivibrator, both of output stats are at stable. So to Changes output from one state to other, we4 should apply the trigger from outside. The output remains in sane state if external trigger is not applied. This bistable muultivibrator is also called flip-flop circuit. A monostablemultivibrator has two output states, only one of which is stable. Its normal mode of operation is to trigger the circuit into its quasistable state, where it will remain for predetermined length of time. The circuit will then switch itself back into its stable state, where it will remain until it receives another input trigger pulse.

Uses of multivibrators

Some of their uses are:

• As frequency dividers
• As saw tooth generators
• As square wave and pulse generators
• As a standard frequency source when synchronized by an external crystal oscillators
• For many specialized uses in radar ad TV circuit
• As memory elements in computers

References:

(1)Theraja, B.L. Basic Electronics. N.p.: S.Chand, n.d. Print.

(2)C.L.Arora. Refresher Course in Physics. Vol. II and III. N.p.: S.Chand, 2006. Print.

(3)Malvino. Electronic Principles. N.p.: Tata McGraw-Hill, n.d. Print.

(4)N.Nelkon and P.Parker. Advanced Level Physics. 5th ed. N.p.: Arnold Heinemann, n.d. Print.

(5)Priti Bhakta Adhikari,Diya Nidhi Chaatkuli, Ishowr Prasad Koirala. A Textbook of Physics (2nd Year). N.p.: Sukunda Pustak Bhawan, 2070. Print.