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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In the monostable multivibrator, one resistive-capacitive network C aztable -R 3 in Figure 1 is replaced by a resistive network just a resistor.

It is triggered by zero or negative input signal applied to Q2 base with the same success it can be triggered by applying a positive input signal through a resistor to Q1 base.

Although the amplifier restores the lost energy when it amplifies the signal, the signal will eventually be too small to be of any use.

What is an Astable Multivibrator?

Time bases scanning generators: The Web This site. Leave this field empty. This mkltivibrators action produces the collector and base waveforms shown in Fig. The circuit is usually drawn in a symmetric form as a cross-coupled pair. An astable multivibrator is a multivibrator that does astablr rest in an unstable state like other multivibrators, but continuously switches between two states.

Thus, Set is used to “set” Q1 on, and Reset is used to “reset” it to off state.

This allows the astable multivibrator to switch between a high state and a low state on a continuous cycle. For example, if Q2 is on and Set is grounded momentarily, this switches Q2 off, and makes Q1 on.


Assume all the capacitors to be discharged at first. As a result, Q2 gets switched off. To approach the needed square waveform, the collector resistors have to be low in resistance. The pulse width T of a monostable multivibrator is calculated as follows: As the charged capacitor discharges and transfers its energy to the second capacitor, the second capacitor recharges and prepares to discharge the energy back to the input.

Both will not be in same state at the same time. It is a type of oscillator and can be used as a trigger, converter, moderator, or divider. It has no stable state but only two quasi-stable states between which it keeps oscillating continuously of its own accord without any external excitation.

A single pair of active devices can be used to divide a reference by a large ratio, however, the stability of the technique is poor owing to the variability of the power supply and the circuit elements. AF Sine Wave Oscillators 4. What is a Loop Antenna? Operational Amplifiers, 2nd Ed. This is due to resistance within the circuit, lack of a completely closed loop at the output terminals, and the tendency for one capacitor or transistor to absorb energy at a slightly different rate than the other.

Thus collector loads of the two stages are equal i.

Astable multivibrators are also used in morse code generators, timers, and systems that require a square wave, including television broadcasts and analog circuits. The circuit essentially consists of two symmetrical CE amplifier stages, each providing a feedback to another.

Other applications included early television systems, where the various line and mulrivibrators frequencies were kept synchronized by pulses included in the video signal.


The basic bipolar transistor BJT version of an astable multivibrator as shown in Fig. As its left-hand negative plate is connected to Q1 base, adtable maximum negative voltage – V is applied to Q1 base that keeps Q1 firmly off. Q2 is quickly driven to saturation. When the voltage of C1 right-hand plate Q2 base voltage becomes positive and reaches 0.


Please help improve this section by adding citations to reliable sources. It is a predecessor of the Eccles-Jordan trigger [7] which was derived from the circuit a year later.

Therefore a 5V supply can be recommended for greater reliablity. Beause Mjltivibrators is turned off at this time, its collector will be at supply voltage and its base will be at almost zero potential, the same as TR1 collector, because C1 is still un-charged multovibrators its two plates are at the same potential. This works via a transistor or amplifier that amplifies the output signal and forwards the charge to the input.

The square wave output can be taken from Q1 and Q2.

For this reason Abraham and Bloch called it a multivibrateur. Ashable the charging capacitor equation above, substituting:. The charging current for C2 is now supplied by R5 instead of R6. Astable multivibrators are used in amateur radio equipment to receive and transmit radio signals.