Multivibrators and their working principle
An electronic circuit that generates square waves is called a multivibrators.
Principal
Every multivibrator essentially has two transistor each of them act as an electronic switch. The two transistor are fully in the off or on state alternatively.devmesh ke idhar in off or on position for any length of time but cannot stay in between this position.
to understand the function of a multivibrator consider its block diagram shown in figure it is a two stage amplifier with 100% positive feedback. The output can be taken across either transistor q1 or q2.
Types of multivibrator
as discussed in the preceding article A multivibrator is a two stage amplifier with output open stage feedback to the input of the other.the two transistor are operating in such a way that they change their state from onto of alternatively after a specific time determined by their associated circuitry. The two possible is state of a multivibrator are..
First stage Q1-ON. ; Q2-OFF
Second stage. Q1-ON ; Q2-OFF
according to the manner in which the two-stage interchange their state the multivibrator may be classified as
- Astable aur free running multivibrator
- Monostable or one-shot multivibrator
- Bistable aur flip flop multivibrator
A stable multi vibrator
multivibrator that generate square wave of its own without any external triggering pulse is known as a stable of free running multivibrator.
Circuit arrangement
Figure shows the circuit arrangement of a commonly used astable multivibrator. The circuit consists of two stage RC coupled amplifier in CE configuration. There are two identical transistor q1 and q2.the output of the first stage is coupled to the input of 2nd stage through a capacitor C1 and the output of the second stage is coupled to the input of the first stage through a capacitor C2. Since the phase of a signal is reversed when amplified by a single stage of CE amplifier, it come back to its original face when pass through two stages.
when supply is given to the circuit both the identical transistor start conducting because of a small difference in their operating characteristic one of the transistor conduct slightly more than the other. Day starts a series of event as explained below.
Latest resume arbitrarily that transistor q1 conduct more than q2. It mean that the collector voltage of q1 drop more rapidly than the collector voltage of q 2. The decrease in BC is applied to the network C1 rb2 and because the charge on C2 cannot change instantaneously the full negative going charge appear across rb2. This decrease the forward bias on q2 which decrease the collector current of q2. Thus, the collector voltage of q to rises. This increased voltage is applied to the base of q1 through the network C2 rb1 which increase its forward bias. Hence,transistor q1 conduct even more heavily and its collector voltage drop more rapidly. This negative going charge coupled to the base of q2 further decreases its collector current.the entire process is commutative which bring transistor q2 entirely in cut of state and q1 in saturated state.
When transistor q2 is in its cutoff state,its collector voltage practically becomes equal to supply voltage vcc and capacitor C2 charge rapidly to BCC through a low resistance path from emitter to base of the conducting transistor q1. When the circuit action turn q1 fully on its collective voltage drop to almost zero volt and since the charge on C1 cannot change instantaneously the base of q2 is at vcc potential and the transistor q2 is driven deep into cut off position.
Now,the switching action start capacitor C1 start to discharge expo 10 asali through Rb2.when the charge on 7:00 reaches zero world it tries to recharge up to the value of day supply. This section produce a forward bias on q2 and this transistor start to conduct.the moment you to start conducting its collector current causes a decrease in the collector voltage Vc2 .
Monostable multivibrator
multivibrator in which one transistor always remain in the conducting on state and the other remains in the non conducting off state is called monostable multivibrator.
Circuit arrangement
the circuit arrangement of a monostable multivibrator is shown in figure the circuit contains two identical transistor q1 and q2 with equal collector load resistance Rc1 and Rc2 respectively. The resistor R1,R2R3 provide the necessary biasing. The transistor q2 is reverse biased to cutoff with the help of separate supply Vbb.the collector supply vcc and R2 forward bias and keep it at saturation. Capacitor 7 provide the necessary positive feedback to q1 where is C2 is employed to feed input impulse to q1.
Operation
When supply vcc is given to the circuit as arrangement in figure transistor q2 is cut off by the reverse bias and q 1 is conducting on state. No negative pulse is applied to the base of Q1 through C2. it is reverse bias and therefore, the collector current of Q1 decreases.this make the collector of q1 more positive and has the base of q2 to attend positive potential. Under such condition transistor q2 is forward biased and start conducting.this makes its collector negative since the base of q one is connected to the collector of q2 thrghgough C1 therefore, base of Q1 become more negative, thus further reducing collector current of Q1.this sequence of event goes on till Q1 is cutoff and q2 is turned on.this sequence goes on as long as the external pulses applied.when the input impulse is removed or reduced to zero the circuit come back to original condition Q1 is fully on and q2 is cutoff.
Bistable multivibrator
multivibrator that has both the state as a stable is called a bistable multivibrator.
Circuit arrangement
the circuit arrangement of a bistable multivibrator is shown in figure the circuit contains two identical transistor Q1 and q2 with equal collector load resistance rc1 and rc2 respectively. Register r3 and R4 provide the necessary by Singh where as R1 and R2 are the register through which the feedback of one stage is coupled to the other stage. The to register R1 and R2 are shunted by the capacitor C1 and C2 respectively. This capacitor improve the switching characteristics of the circuit bypassing the high frequency component of the square wave. Dzire low fast rise and fall times and hands distortion is removed from the output is square wave.
Operation
When supply vcc is applied to the circuit one transistor conducts slightly more than the other q2. Due to some difference in their characteristics this will drive q1 in on saturation state and q2 in off cut off estate in fraction of second. The circuit will stay in this condition if it is left as it is. In order to switch the circuit to its other is 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 cutoff.
once a pulse is trigger to the base of q1 or q2 mentioned above,the forward bias on q1 reduced which in turn decrease its collector current and increase the collector voltage. This rising collector voltage is coupled to the base of q2 where it forward by the base emitter junction of q2. This will increase the collector current of q2 and cause a decrease in its collector voltage which is coupled to the base of Q1. This further reverse biasing the base emitter junction of q1 and decrease its collector current. This community process bring to q1 cut off state and q2 to saturation on state.
The circuit against a in the position until further trigger pulse is applied. Thus,after every operation pulse has to be applied to change the state of the two transistor. It is also called the flip-flop as it's a in the same state till next person is applied. It is used in digital memory device.
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