Types and classified of multi-vibrator

                       Types of Multi-vibrator

As a discussed in the preceding article, a multi-vibrators is a two-stage amplifier with output of one stage feedback to the input of the other. The two transistor are operating in such a way that they change there state from ON to OFF alternately after a specify time determined by their associated circuitry. The two possible states of a multi-vibrator are: 

According to the manner in which the two stages interchanging their, the multi-vibrator may be classified as:

1. A-stable multi-vibrator

2. Mono-stable or one-shot multi-vibrator

3. Bi-stable or flip-flip multi-vibrator

1. A-stable multi-vibrator

A multi-vibrator that generates square wave of its own without any external triggering pulse is known as a-stable multi-vibrator.

A-stable multi-vibrator

Operation of A-stable multi-vibrator:-When supply is given to the circuit both the transistor start conducting. Because of a small difference in their operating characteristics, one of the transistor conducts slightly more than the other.

Let as assume arbitrarily that transistor Q1 conduct more than Q2 it means that the collector voltage of Q1 drops more rapidly than the collector voltage of Q2. The decrease in Vc1 is applied to the network C1 RB2 and because the charge on C2 cannot change, the full negative going charge appears across RB2.

When transistor Q2 is in cut-off region its collector voltage practically becomes equal to supply voltage Vcc and capacitor C2 charge rapidly to Vcc through a low-resistance path from emitter to base of the conducting transistor Q1.

2. mono-stable multi-vibrator:-

A multi-vibrator in which one transistor always remains in the conducting [ON] state and the other remains in the non-conducting[OFF] state is called a mono-stable multi-vibrator.

Operation:-When supply Vcc is given to the circuit as arranged in fig transistor Q2 is cut-off by the reverse bias and Q1 is forward bias is conducting[ON state]. Now negative pulse is applied to the base of Q1 through C2, it is reverse biased and therefor, the collector current of Q1 decrease. This makes the collector of Q1 more positive and hence the base of Q2 attains positive potential. Under such condition, transistor Q2 is forward biased and start conducting. This makes its collector negative. Since the base of Q1 is connected to the collector of Q2 through C1 therefore, base of Q1 become more negative, Thus further reducing collector current of Q1. This sequence of event goes on till Q1 is cut-off and Q2 is turned ON.

  3. Bi-stable multi-vibrator:-

A multi-vibrator that has both the states as stable is called bi-stable multi-vibrator.

In a bi-stable multi-vibrator, both the transistor have stable states. both the transistor will remain in the same state in which they happen to be untill a pulse causes them to switch to the other state.

Operation:-

When supply Vcc is applied to the circuit, one transistor conduct slightly more than the other Q2 due to some difference in there characteristics. This will drive Q1 in on [saturation] state and Q2 in OFF [cut-off] state fraction of second. The circuit will stay in this condition if it is left as it is . In order to switch the circuit [multi-vibrator] to its order state either we have to trigger a positive pulse to the base of Q2 through C4 which will cause it to conduct or we have to trigger a negative pulse to the base of Q1 through C3 which will cause it to cut-off.

Once a pulse is trigger to the base of Q1 or Q2 as mentioned above, the forward bias on Q1 reduce which in turn decrease its collector current and increase the collector voltage. this rising collector voltage is coupled to the base of Q2 where if forward bias the base-emitter junction of Q2. This will increase the collector current of Q2 and cause a decease in its collector voltage which is coupled to the base of q1. THis further reverse bias the base-emitter junction of q1 and further decrease its collector current. This cummulative process brings Q1 to cut-off[OFF] state and Q2 to staturation [ON] state.

The output wave-shape of this multi-vibrator is shown in fig it may be seen that the output is distortion free square wave because of the shunting capacitors C1 and C2.

• Tweet
• Share
• Share
• Share
• Share

About Admin MC3

This is dummy text. It is not meant to be read. Accordingly, it is difficult to figure out when to end it. But then, this is dummy text. It is not meant to be read. Period.

Related Post