After Studying this section, you should be able to: • Recognise BJT astable multivibrator circuits. • Understand the operation of astable multivibrators. • Calculate. An Astable Multivibrator or a Free Running Multivibrator is the multivibrator which has no stable states,oscillates continuously between two unstable states. A multivibrator is a device that switches between two states. It is a type of oscillator and can be used as a trigger, converter, moderator, or divider. It.

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Design and setup an astable multivibrator and calculate the time period and duty cycle of the output waveform. Multivibrator is considered as a non-sinusoidal oscillator which can generate waveforms other than sine wave.

They are basically two-stage amplifiers with positive feedback from the output of one amplifier to the input of the other. This feedback is supplied in such a manner that one transistor is driven to saturation ON state and other to cut-off OFF state.

The action is reversed after a certain time, depending upon the circuit conditions i. There are three basic types of multivibrators.

What is an Astable Multivibrator?

It is also called free-running relaxation oscillator. It has no stable state but only multivibratkrs quasi-stable states between which it keeps oscillating continuously of its own accord without any external excitation.

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Both will not be in same state at the same time. The circuit essentially consists of two symmetrical CE amplifier stages, each providing a feedback to another. Thus collector loads of the two stages are equal i. The output of transistor Q1 is coupled to the input of Q2 through C1 while the Q2 is fed to the input of Q1 through C2. The square wave output can be taken from Q1 and Q2. Because of very strong feedback signal, the transistors are driven either to saturation or to cutoff.

When Vcc is applied, collector current start flowing in Q1 and Q2. In addition, the coupling capacitors C1 and C2 also start charging up. The rising collector current in Q1 drives its collector more and more positive. This positive output is applied to the base of transistor Q2 through C1.

Module 4.1

This establishes a reverse bias on Q2 and its collector current starts decreasing. As the collector of Q2 is connected to the base of Q1 through C2, therefore the base of Q1 becomes more negative i.

Q1 is more forward biased.

This further increases the collector current in Q2 and cause decrease of collector current Q2. This is repeated until the circuit drives Q1 to saturation and Q2 to cutoff. Therefore full voltage Vcc appears across Rc1 and voltage across Rc2 will be zero.


The charges developed across C1 and C2 are sufficient to maintain the saturation and cutoff conditions at Q1 and Q2 respectively. However, the capacitor will not retain the charges indefinitely but will discharge through astwble respective circuits. As C1 discharges, the base bias at Q2 becomes less positive and at a time determined by R1 and C1, forward biased is re-established at Q2.

This causes asatble collector current to start in Q2. The increasing positive potential at collector of Q2 is applied to the base of Q1 through capacitor C2.

BJT Astable Multivibrators

Hence the base of Q1 will become more positive than Q2. Q2 is awtable driven to saturation. The period of time during which Q2 remains at saturation and Q1 at cutoff is determined by C2 and R2.

The resistors R1 and R2 must be able to provide base current enough to keep the transistors in saturation. Bistable multivibrator Astable multivibrator It is also called free-running relaxation oscillator.

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