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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The width of the output pulse depends only on external components connected to the op-amp. Hence, when the circuit is switched on, if Q1 is on, its collector is at 0 V.

Please help improve this section by adding citations to reliable sources. In practice, oscillation always occurs for practical values of R and C. Because they do not need to be the same, an asymmetric duty cycle is easily achieved.

A change of state has occurred at both outputs. The frequency of oscillation can be calculated, as the time for the relevant capacitor to charge sufficiently for a change of state to take place, will be approximately 0. 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. January Learn how and when to remove this template message.


Suppose that at switch on, TR1 is conducting heavily and TR2 is turned off. When the voltage of C1 right-hand plate Q2 base voltage becomes positive and reaches 0. The circuits shown on this page will operate from a DC supply between 3.

This establishes a reverse bias on Q2 and its collector current starts decreasing. The charges developed across C1 and C2 are sufficient to maintain the saturation and cutoff conditions at Q1 and Q2 respectively.

There are three basic types of multivibrators. While not fundamental to circuit operation, diodes connected in series with the base or emitter of the transistors are required to prevent the base-emitter junction being driven into reverse breakdown when the supply voltage is in excess of the V eb breakdown voltage, typically around volts for general purpose silicon transistors.

The voltage at the non-inverting terminal will be greater than the voltage at the inverting terminal of the op-amp.

Views Read Edit View history. It is a type of oscillator and can be used as a trigger, converter, moderator, or divider.

The Web This site. Q2 begins conducting and this starts the avalanche-like positive feedback process as follows.

What is an Astable Multivibrator?

This feedback is supplied in such a manner that one transistor is driven to saturation ON state and other to cut-off OFF state. However, the capacitor will not retain the charges indefinitely but will discharge through their respective circuits.


The voltage at inverting terminal will be greater than the voltage at the non-inverting terminal of the op-amp. It is considered below for the transistor Q1.

BJT Astable Multivibrators

After elapsing the time, it returns to its stable initial state. A multivibrator is an electronic circuit used to implement a variety of simple two-state [1] [2] [3] devices such as relaxation oscillatorstimers and flip-flops.

As its left-hand negative plate is connected to Q1 base, a maximum negative voltage – V is applied to Q1 base that keeps Q1 firmly off.

A free-running multivibrator with a frequency of one-half to one-tenth of the reference frequency would accurately lock to the reference frequency. This page was last edited on 20 Novemberat Thus, Set multivibrrators used to “set” Q1 on, and Reset is used to “reset” it to off state.

Module 4.1

To approach the needed square waveform, the collector resistors have to be low in resistance. Got Something To Say: Operational Amplifiers, 2nd Ed. What is an Astable Multivibrator?

This positive output is applied to the base of transistor Q2 through C1.